I was always under the assumption that it was legal, or at least no more illegal than any other in German history. But apparently there has been contention on this point, as it goes, Hitler supposedly was thrust into power by Franz von Papen and his conservatives with the backing of Hindenburg against the chancellorship of Kurt von Schleicher.
The newest major Hitler biographer Volker Ullrich says the following in a throwaway comment
Hitler quickly forgot that he owed his rise to power not to some miracle but to a unique constellation of political crises, and that at the precise moment when his bid for power was about to fail he had been heaved unto the Chancellory by a sinister plot. - Hitler: Ascent by Volker Ullrich, pp. 513
In Ullrich describes in Chapter 12 this meeting 'plot'
The planned meeting opened up interesting prospects for both Papen and Hitler. Papen had not got over being politically outmanoeuvred by his former patron Schleicher and was eager to get revenge, seeing a deal with Hitler as a way of forcing Schleicher from office and once more playing a major role himself. For his part, Hitler recognised that a possible understanding with Papen offered the chance of getting out of the dead end into which he had led his party and reversing his own fortunes. He knew that Papen retained privileged access to Hindenburg and hoped that the aristocrat could help break down the president’s resistance to the idea of Hitler becoming chancellor.13 Both sides insisted that the meeting be kept secret.----Hitler and Papen immediately withdrew for two hours into Schröder’s office, with their host accompanying them as a silent witness. Schröder recalled Hitler opening the conversation by attacking Papen for his government’s handling of the Potempa case. Papen replied that they should put their past differences behind them and try to arrive at a common basis for a new government that would consist of conservatives and National Socialists. It seems that Papen suggested something along the lines of a “duumvirate,” in which he and Hitler would share power. To make this idea more appealing, he dangled the positions of defence and interior ministers before Hitler’s nose, whereupon Hitler commenced one of his feared monologues in which he justified his insistence upon the chancellorship. However, he would accept Papen adherents in his cabinet, as long as they supported the changes he wanted to institute after taking office. The first measures Hitler specified were “the removal of all Social Democrats, Communists and Jews from leading positions” and the “re-establishment of order in public life.” There was still a considerable gap between the two men’s positions, but they did promise to continue their discussions. On 6 January, Keppler wrote to Schröder that “the meeting had a very positive effect in the desired direction.” Hjalmar Schacht, too, thanked the banker for the “courageous initiative in paving the way for an understanding between men whom we both greatly admire and whose cooperation could perhaps bring about a positive solution most quickly.” Hopefully, Schacht wrote, “the conversation in your house will be of historical importance.”16 Schacht’s hope would come true. The meeting in Cologne was the starting point of a process that concluded on 30 January 1933. Hitler, who had been at his wits’ end in the final days of December, saw himself catapulted back into the contest for power. The most important thing to emerge from the Cologne meeting was that Papen and Hitler agreed to put aside their enmity and work together to topple Schleicher.
None of this particularly strikes me as sinister or illegal. But I'll continue
That same day, Papen also reported to Hindenburg about his meeting with Hitler. If we believe Otto Meissner’s memoirs, Papen said that Hitler “had given up his previous demands for the entire power of government and was now prepared in theory to participate in a coalition government with other right-wing parties.” In response, Hindenburg charged Papen with continuing to negotiate with Hitler “on a personal and strictly confidential basis.” With that the Reich president consciously and in full knowledge of the consequences became a participant in a conspiracy aimed at creating a new government of “national concentration” behind the current Reich chancellor’s back It was the same sort of government that Hitler’s intransigence had blocked in the autumn of 1932.
He goes on to describe that Schleicher's popularity with the trade unions and the aristocratic landowners has plummeted to non-existence, that he had a falling out with Oskar von Hindenburg and he didn't have the full support of the Reichstag. This all seems quite apart from any 'sinister plot'.
Schleicher was no longer assured of the Reich president’s support. As early as 10 January, Goebbels learned that the chancellor could by no means count on an executive order dissolving parliament if the Reichstag staged a vote of no confidence when it reconvened in late January. Schleicher, Goebbels noted, was on “a downward slope.”26 And indeed, the chancellor’s position in early January was precarious. In a government statement he had made via radio on 15 December 1932, Schleicher had presented himself as a “socially responsible general,” promising not just to take measures to boost employment, but also to revoke the ordinance of the Papen cabinet from 5 September that allowed employers to pay lower wages than those contained within the official labour agreements. These announcements alienated business leaders, and Schleicher further awakened mistrust with the “sacrilegious” assertion that he supported “neither capitalism nor socialism” and that he was not impressed by concepts like “private or planned economy.”27 On the other hand, Schleicher never succeeded in garnering the support of trade unions. While his policy of state-financed job creation met with approval in union circles, the Social Democrats, who were closely allied with the unions, stuck to their position of “absolute opposition” to the chancellor, whom they rightfully accused of being in part responsible for the coup d’état in Prussia in July 1932.28
Any hopes Schleicher had placed in cooperating with Gregor Strasser proved unrealistic. On 6 January, the Reich chancellor introduced the former NSDAP organisational director to Hindenburg, who agreed in principle to appointing Strasser vice-chancellor and labour minister. But Schleicher did not force the issue. Once news had got around about Papen and Hitler’s Cologne meeting, there was very little chance that a significant part of the NSDAP would support Schleicher’s government.29
To make matters worse for Schleicher, his cabinet was coming under attack from the Reichslandbund, the lobbying organisation of Germany’s wealthy aristocratic landowners. It accused the government of not doing enough to protect big farmers against cheap food imports and of foreclosing on bankrupt agricultural enterprises. On 11 January, Hindenburg received a delegation from the Reichslandbund, consisting of four committee members, which included the National Socialist Werner Willikens. The organisation’s president, Count Eberhard von Kalckreuth, painted the situation in the blackest terms. If immediate measures were not taken to “improve economic conditions in agriculture,” disaster was imminent. Hindenburg immediately ordered Schleicher and his ministers for agriculture and economics to listen to the complaints and to take action.30 Shortly after this meeting, however, a declaration that the Reichslandbund had given to the press before seeing Hindenburg was published. It accused the government of hastening the “impoverishment of German agriculture” in a fashion “unimaginable even under a purely Marxist regime.” Schleicher responded to this attack by refusing to negotiate in future with representatives of the Reichslandbund.31
In many respects the anti-Schleicher conspiracy resembled the campaign against Brüning’s purported “agrarian Bolshevism” that had led to his dismissal in late May 1932. Hindenburg, who as the owner of a large estate in Neudeck was thoroughly receptive to the concerns of the East Elbian agrarians, was once again easily influenced. Schleicher’s position, already weakened by Papen and the failure of the plans for a “cross-party front,” was further undermined. “Schleicher has a conflict with the Landbund—the farmers are going wild,” Goebbels noted. In a revised version of his diary from 1934, he was even more explicit: “That serves us quite well at the moment.”32 But perhaps even more damaging to Schleicher was the deterioration since late 1932 of his relationship with Oskar von Hindenburg, who served as a military adjutant and close adviser to his father. We do not know what precisely caused the rift between the two men, but the consequences were serious. Schleicher lost his most important advocate in the house of Hindenburg.33
As if the situation were not bad enough, the DNVP also distanced itself from the government. On 13 January, Alfred Hugenberg offered to join the Schleicher cabinet as minister of economics and agriculture, but only on the condition that the chancellor implemented a strictly authoritarian regime that ruled independently of parliament. Schleicher, however, had made clear in his radio address of 15 December that he “could hardly sit on the point of a bayonet” and “could not rule for very long without broad popular approval behind him” and therefore refused Hugenberg’s offer.34 Consequently, a week later, on 21 January, the DNVP Reichstag faction attacked the Schleicher government in tones no less harsh than the Landbund. Schleicher’s government “tended towards internationalist socialism,” the faction declared in a statement. “It runs the risk of Bolshevism in the countryside and is liquidating the authoritarian idea that the Reich president created when he appointed the Papen cabinet.”35
All of this was grist for the mill of Schleicher’s enemies. During the night of 10–11 January, after attending a performance of Verdi’s La Traviata, Hitler met again with Papen----We do not know precisely what Hitler and Papen discussed at their second meeting, but apparently they did not make much progress since Hitler declined at short notice an invitation to continue the exchange of opinions over lunch in Dahlem on 12 January. “Everything still up in the air,” noted Goebbels.37
And again just offhand
Hitler was quite preoccupied by the conspiracy to overthrow Schleicher
The Reich chancellor’s demise was getting closer and closer. “Schleicher’s position is very bad,” Goebbels noted on 22 January. “When will he fall?”73 Two days before that, the Reichstag Council of Elders decided that Germany’s parliament would indeed be called to session on 31 January. Given that only the tiny DVP faction had declared its support for the government, Schleicher was headed for a devastating vote of no confidence, just like Papen before him. ---- on 23 January at a meeting with Hindenburg, who declared that while “he would consider dissolving the Reichstag, he could not take responsibility for postponing the election beyond the deadline specified by the constitution.”75 This decision was hardly a surprise. By resorting to Papen’s old emergency plan, Schleicher was admitting that, just like his predecessor, he had failed to establish a broad parliamentary majority to tolerate his government. Hindenburg found it presumptuous that the chancellor would try to take the same escape route that he had used to drive his predecessor from office. By this point the Reich president seems to have decided to drop Schleicher—a decision made easier since he was being kept informed about the secret negotiations between Papen and Hitler and knew that a potential alternative was emerging.
Anyway, I don't want to end up putting the entire chapter here. But none of this seems remotely sinister, or couplike. It seems like parties and people conversing to get the desire they want, where is the illegality in this?
Others have also written about how Hitler's power was totally legal, particularly H.W. Koch in 'Aspects of the Third Reich' http://booksdescr.org/item/index.php?md5=AE119350B04AA16D4D1E8A9099108F26 and Norman Stone in his short biography of Hitler http://booksdescr.org/item/index.php?md5=BEB711EAA0309447A17AFDA92303E365. But yet the idea still persists in the most modern literature. I'm curious as to what others takes are on this issue. I highly recommend you read Koch
I'd also like to do three other threads, one on the Night of the Long Knives where we can document details, another thread on the Sturmabteilung (SA) and their supposed 'thuggery' and finally on Hitler's popularity and the Volksgemeinschaft.