Well, to start off, you have to go back to the beginnings of the Republic right after WWI. There were several flaws that the Weimar Republic was "born with", among them:

• the fact that it was proclaimed by social democratic politician Philipp Scheidemann as well as socialist Karl Liebknecht, separate from each other within a few hours. Besides giving off a chaotic look from the get-go, this also meant that it was done over the heads of most people in Germany. This proclamation, as you can see came from the moderate to far left political spectrum, which also dominated the next steps of forming a National Assembly. In this process, the "bürgerliche" (moderate-conservative) faction was not very much represented and had very little to say. The left-constellation of socialists and social democrats was very fragile and lasted a mere month, before infights, and open street brawls between the two broke out. So, to sum this point up, the Republic was proclaimed by only a part of the political party spectrum, and very chaotic.

• the signing of the Versailles treaty by the democratic government of SPD (social democrats) and Centrum (the predecessor to the moderate-right, conservative CDU). This treaty was perceived as unfair, harsh and all in all too much of a burden to bear after the war. Most Germans, including the signees, thought that it unfairly placed the sole blame for the outbreak of the war on Germany and its allies. The parties, despite signing under protest later were blamed for signing the document, and under right-wing propaganda, the "stab in the back" legend that Germany could still have negotiated a much better peace treaty, but was stabbed in the back by the Weimar politicians was born out of it. It cost the democratic parties a lot of respect among the population.

• The constitution of the Weimar Republic also allowed for any party, however small to be represented in parliament, which over time led to an assembly of small fractions, broken off of the bigger parties. This meant that forming a government proved more and more difficult, rendering the parliamentary system ineffective, and in turn shedding a bad light on the democratic processes in Germany. This again gave rise to anti-democratic parties such as the communists and ultimately the Nazis.

Another factor in the decline of the Republic was that the governments formed had to rely on many of the old personnel from the Kaiserreich era, who were far from supportive of democracy. In fact, while entrusted with keeping stability and order, they at the same time sympathized with anti-democratic groups, as well as accepted paramilitary troops formed by disgruntled war veterans as auxiliary police troops. Those "freikorps" veterans were often monarchists or at least hostile to democracy as well, and they made sure to put pressure on communists, socialists, social democrats and liberal democrats.

All of these factors led to an ineffective government, a climate of political unrest, and the view that democracy could not handle the state's affairs properly. add to that some other crises such as the economic crisis of the 1920s and early 1930s, and the grounds for the rise of the Nazi party were prepared.

The Nazis then exploited some loopholes in the Weimar constitution, such as the enormous power of the Reichspräsident to dissolve parliament at will. They would block sessions of parliament through walk-outs and thus effectively shut down all parliamentary procedures, which the (monarchist) president Paul von Hindenburg then took as a reason for dissolving parliament and proclaiming new elections. This led to a constellation where the "bürgerliche" fractions formed a government with the Nazis in 1933, making Hitler the Chancellor. From then on, Hitler and his party assumed dictatorial powers after blaming the Reichstag fire on the communists, and thus had the means to silence all other dissenting political powers.