It had a promising beginning:
A constitution guaranteeing federal rights, seven year presidential terms and proportional representation was passed in Weimar on the 19th July 1919.
This should have meant a good foundation for democracy and sound government. Article 48 stopped (in theory) the major politicians from arguing. It was supported by the Big 3 world Superpowers (Great Britain, USA and France). The Constitution meant that everyone as politically accountable within quite an advanced political structure which seemed to promise true democracy.
And yet it was also based on flimsy foundations.
It had accepted the validity of the hugely unpopular Versailles Treaty which ended the First World War this meant that it accepted as law the large amount of reparations imposed on Germany as instigator of the conflict. This led to financial and economic weakness underpinned by political unpopularity with the people. It was associated with defeat and dishonour from the start.
There was a tradition in Germany of autocratic leadership from the Officer class. The Army had put it about that it had felt betrayed by the weak' democrats and bureaucrats who had sold out the army at the end of the war. The Treaty and the Weimar Constitution was the public face of this.
Proportional Representation (PR) was a good idea in theory as it guaranteed all the participating political groups an equal say in decision making, but the reality was that there were too may small groups and there result was chaos. No one party could ever achieve a working majority. A succession of coalition governments led to in-fighting and accusations that the system wasn't working.
The political parties had little experience of how to run government. Traditionally the Chancellor held final power but under the new constitution he had to report to the Reichstag who in turn was divided and leaderless. The Communists and Conservatives/Nationalists (the extremes) did not believe in the Republic anyway and therefore did not attempt to make it work. Each party/movement set up its own private militia and this in turn led to greater distrust.
The consequence of this was that there were outbreaks of violence which the Government was incapable of putting down. For example, Jan 1919 the Communist Sparticist Uprising led by Karl Liebknecht and Rosa Luxembourg. The Kapp Putsch in March 1920 was an attempt by the Right Wing to seize power. A series of political assassinations of key political figures, notably of Walter Rathenau and Gustav Erzberger by ex-Freikorps members also caused serious unrest and eliminated potentially strong leaders. The French occupation of the Ruhr in November led to public unrest and to the Munich Beer Hall Putsch in which a young ex-army Corporal from Austria, Adolf Hitler attempted to seize independence in the Bavarian capital, but was crushed. It was a sign of things to come, though.
The period between 1923-29 was quieter, but as unemployment rose in the early 30s and the economic climate worsened, the symptoms of unrest worsened.
Other economic factors intervened:
The near economic bankruptcy brought on by the First World War; the failure to be able to meet reparations payments after the £50 million due in 1921; the huge inflation brought on by the French invasion of the Ruhr in 1923 and the industrial resistance that ensued. 4 arks to the dollar became 4.2 million by 1923. This hit the working and middle classes very badly, but the rich landowners and industrialist who had their wealth in land and property manager to insulate themselves, causing even greater social friction.
The Dawes Plan in 1924 made large improvements, reducing reparations to one-third and a $40 million loan from the USA helped.
However, this placed the German economy in the hands of the Americans and if the US economy faltered as it did in 1929, then the impact would be twice as strong in Germany.
The Catholic Centre Party Government of Chancellor Bruning adopted good measures to improve the economy, but was unable to stop unemployment rising to 6 million by 1932, creating an environment where other factors could cause its downfall.
If nothing else had happened, it is possible the Weimar Republic might have survived, whilst being unpopular, but the period from 1932 saw the growth in the popularity of the Nazi Party under Hitler, who appealed directly appealed to the groups in society worse hit by the depression.
The Nazis offered national unity; promised to dispose of the Versailles settlement; create a wider Germany including Germans in Poland, Austria and Czechoslovakia; the private army, the SA offered an attractive destination for disenchanted young people - and wealthy landowners and industrialists supported the Nazis as a strong alternative to Communism. Hitler himself was a hugely popular and charismatic figure with advanced political abilities. The contrast with the dull, worthy Weimar politicians was stark.
A right wing clique of politicians decided Hitler was better off inside the tent than outside and invited him into coalition. Papen, Schleicher and Hindenburg were responsible for underestimating Hitler's will and the economic forces that made his message so potent.
In conclusion, the downfall of the Weimar Republic economically and politically was inevitable. However, the rise of Hitler and the domination of the Nazi Party for the next 12 years could have been avoided with more decisiveness amongst other politicians and more support from overseas. The window of opportunity that Depression around the world offered, enabled Hitler to finally kill off the Weimar Republic.