One can point to iconic moments which completely transformed history: the 9/11 bombings, the creation of the internet and Watergate, arguably none more so than Hitler's accession to chancellorship. The events of 1933 gave rise to a debate which would dictate the trajectory of the 20th century- how did the leader of a fascist fringe party, Adolf Hitler, become one of the most prominent members in German, and world history? It's a question which has dominated debates in the field of history, politics and international relations for much of the 20th and 21st century, and is vital to our understanding of the forces responsible for instability today. The general consensus is one cannot simply cite a monocausal explanation in assessing the reasons for Hitler's elevation to the station of chancellorship. While party popularity played a key role in his appointment, pressure from the right wing, political intrigue, his personal appeal and the failure of mainstream politics contributed significantly to the eventual decision taken by Hindenburg to appoint Hitler as chancellor.
Popularity of the Nazi party
Clearly the unprecedented rise of Nazi popularity in the years preceding Hitler's appointment was a considerable factor in the subsequent unfoldment of Hitler's rise to power. It's important to note that the Nazi party from gaining 2.6 % of the electorate in May of 1928, became the largest party in Reichstag in July 1932 as they gained 37.5 % of the vote. Hitler's political positioning of the party as a "volkspartei" (the people's party), had become a worrying reality. He had manoeuvred the party in a way so as to transcend traditional political ties of religion and class and appealed to a broad spectrum of society and gained support from all sections of society. Thus by 1932 the Nazi party had unequivocally established themselves as a 'peoples party'. However the role of their popularity shouldn't be overestimated, as while they were the largest party in the Reichstag, this only influenced Hindenburg's decision. Because under the Weimar constitution the president had sole responsibility to appoint the chancellor. Clearly popularity of the party contributed greatly to Hindenburg's decision as Hitler's popular appeal, in theory, would make for greater support for the chancellor. However clearly popularity didn't fully account for his appointment. While they were the largest party in July of 1932 Hitler wasn't appointed then, and by November of the same year their support reduced to 33%, thus Hindenburg's decision to delay Hitler's appointment until January 1933 indicates popularity wasn't the primary driving force behind Hitler's appointment, an idea which is reinforced by the fact the Nazis never gained a majority. Although, evidently the popularity of the Nazis provided the initial impetus for Hitler's appointment, the constitution gave the President power to appoint a chancellor independent of the Reichstag, and thus popularity of the party doesn't fully account for his appointment.
Pressure from conservative elites
Alongside pressure from the popular vote Hindenburg was simultaneously burdened with pressure from right wing conservative elites, who urged the appointment of Hitler as chancellor. Conservatives in big business, and high ranking officers in the armed forces, for example, overcome by fear of communism and the rising force of the KPD allied their forces with the violent anti communist message propagated by Hitler and the Nazi party. To the forces of conservatism the threat of communism was seen as an acute threat; their vote increased from 3.2 million in 1928, to 5.9 million in 1933. This continuous "red threat" which had supposedly plagued German society since the inception of the republic, led industrialists, bankers and corporations like Hjalmar Schacht, IG Farben and Gustav Krupp to put pressure on Hindenburg, emphasising their belief in Hitler's ability to dispel the threat from the revolutionary left. This is reinforced by the party donations made by prominent industrialists. While the pressure applied by the armed forces was driven by their fears of extremist uprisings and their inability to deal with the threats, thus with Hitler as chancellor the government would control the SA, and thus alleviate the threat to the army.
While the pressure from the leading members of German society contributed to Hitler's appointment, it was contingent on anti-communist Nazi ideology and Hitler's personal appeal. At a time of weak politicians, Joseph Goebbels, Hitler's 'propaganda man', portrayed Hitler as Germany's heroic saviour, particularly cultivated in the 1932 presidential election, at which point "the Hitler myth" became firmly imprinted on German minds. During the election campaign; "Hitler over Germany", he was portrayed as statesman like figure, particularly due to his use of modern technology, such as travelling fly aeroplane to campaign. His personal appeal was, arguably most potently evidenced by his unique, evocative and captivating speaking style, and his talent as an orator likewise contributed to the image and personal appeal associated with Adolf Hitler. By this point Nazi ideology had become well established in Germany and his violent anti communistic, anti semitic and anti Weimar attitudes, accentuated by the Great Depression, served to boost his popularity but also contribute to his appeal to leading members of German society. Thus Hitler's personal appeal was a significant factor, and made it a much easier task for the right wing elites to convince Hindenburg to appoint him.
Failure of mainstream politics
Hitler's personal appeal has to be put into the context of the failure of mainstream politics, and thus cannot be alone held accountable for his appointment. Since its inception the Weimar Republic had been plagued by the Treaty of Versailles, which was construed by the right wing as a 'diktat', and these anti-Republic sentiments continue to dominate right wing propaganda, namely Nazism. However throughout the Great depression, and in its aftermath the weakness of Weimar heightened the publics lack of faith in the Republic, thus a greater popularity in the anti-Weimar sentiments of the right wing. Politicans didn't take effective action to deal with the Depression, it was only in 1932 when modest reflationary methods begun, in turn causing irrevocable damage to public faith in the Republic. Simultaneously, to further damage support for the Republic this period faced a considerable retreat from parliamentary democracy, and suffered a breakdown of law and order. After the collapse of the "Grand Coalition" in 1930, a period of instability followed as democracy became more authoritarian, as both Bruning and Von Papen increasingly ruled using the Emergency decree (Article 48 of the constitution). As a comparison, in 1930 the decree was used only 5 times, while in 1931 it was used 44 times, which indicates the inability of the government to gain Reichstag support, let alone the support of the public. The use of article 48 to seize control of the left wing, SPD, government in Prussia epitomises the level to which Weimar had moved towards authoritarian rule. In conjunction with the move towards authoritarianism came a breakdown in law and order, most potently shown amidst the chaos of the 1932 Reichstag election, during which there were 461 riots in Prussia, predominantly instigated by the SA; which contributed to an atmosphere of discontent with the Republic. The move towards authoritarianism served to undermine democracy, but created an environment in which Hitler's authoritarian approach could be easily accommodated. While the failures of mainstream politicians served to reinforce the long term discontent which had existed against Weimar, thus to an extent the failure of mainstream politics was a significant element in bringing about Hitler's accession to chancellor.
Right wing political intrigue
Clearly the weaknesses of Weimar served to augment the atmosphere of discontent, creating an environment of authoritarian rule, however right wing political intrigue acted as, arguably the most direct link between Nazi popularity and Hitler's eventual appointment. Subsequent to Von Papen's government was dismissed, and he lost his office to Kurt Von Schleicher he, naturally, had the desire to regain office and in January 1933 held a meeting with Hitler, in which it was agreed Hitler would proceed as chancellor and Von Papen as his vice, the idea being he would use Hitler's popular support to give legitimacy to his government. This coincided with Von Schleicher's inability to split the Nazi party, as his appointment of Strasser simply isolated him from the Nazi party, and this Von Papen was able to convince Hindenburg to appoint Hitler as chancellor, who did so 30th January 1933. Thus right wing political intrigue provided the final impetus in Hindenburg's decision to appoint Hitler as chancellor.
Evidently one cannot merely attribute a monocausal explanation to an issue of such complexity, each factor contributed to the construction of an environment, in which, the appointment of Hitler as chancellor became increasingly plausible. However the fundamental lack of support for Weimar, accentuated by the economic problems of the early 30's, served to heighten the influence of Hitler's personal appeal, while right wing political intrigue ultimately led to his appointment. Thus it could be argued that both the failure of mainstream politics and the final stage, right wing political intrigue were the most significant elements in the eventual decision to appoint Hitler as chancellor.