From an earlier answer of mine
Most of what remained of the opposition to the Hitler dictatorship in the rump Reichstag that voted for the Enabling Act were what have been called the liberal parties and the Centre, as KPD deputies and the SPD leadership had been imprisoned by the Reichstag Fire Decrees. While these liberal and Catholic leaders did not relish the Act's provisions, they were able to rationalize it as a necessary measure and one that would not bite them in the end.
The Centre's motivations for voting for the Act are somewhat unclear and explanations often revolve the figure of Prelate Ludwig Kaas in the negotiations as a three-way conduit between Hitler, the Centre, and the Vatican. The more extreme forms of this interpretation such as Goldhagen's A Moral Reckoning hold that the Vatican via Kaas sold out the Centre in exchange for a Concordat with the Papacy, which was being negotiated simultaneously with Germany. Hitler's promises to Kaas of preserving Church independence as well as wider notions of creating a European-wide anticommunist conservative front appealed to the Vatican far more than propping up the ailing Centre party. Other historians suggests that Kaas was less of the Vatican's catspaw but rather worked in alliance with the Centre to make a deal with Hitler in exchange for certain political favors. Kaas, along with the former Centre Chancellor Heinrich Brüning hammered out these negotiations with Hitler and forced reluctant Centre deputies in the Reichstag to swallow the bitter pill and pass the Act. These deals ranged from protection of Catholic institutions like schools and hospitals to guarantees that the new state would not purge Catholics from the civil service.
Kaas and the Centre at least had the comfort of working from a position of relative strength. The same could not quite be said of Germany's liberal parties. The larger trajectory of weakening political power of these liberal parties defined much of German liberals' responses to the rise of Hitler, which ranged from disgust and rejection of the NSDAP to the cooption. Liberals within the more left-wing orientated DDP tended to place themselves in opposition to Hitler between 1929-33, but it is important to realize that German liberals were often not opposing Hitler per se, but rather a wider turn in German politics towards extremism both on the left and far-right.
Yet German liberals were not above the wider radicalization process that transformed Weimar politics in the 1930s. Many of the DDP and DVP intellectuals embraced various völkisch ideas developed during the war, and often saw the rise of communism as a greater threat to Germany than Hitler's NSDAP. This ideological overlap with some of the tenets of National Socialism made it much more difficult for liberals to form political coalitions with either the SPD or Centre parties in opposition to Hitler and the various conservative political parties seeking to end the Republic.
The result was that the picture of liberals in Germany was either of a doomed rearguard against the rise of Hitler or a cowardly surrender to the Nazis. But liberals' responses to various Nazi political initiatives was often quite complex, and elides simple notions of last stands or surrender. The position of various liberals in the passage of Enabling Act in 1933 was emblematic of this multifacted response to the rise of the dictatorship. With one lone exception, all of the DStP (a successor of sorts to the DDP) delegates in the Reichstag voted for the Enabling Act, a seemingly about-face to liberal democratic principles.The rationale for supporting the Enabling Act varied among the deputies. Some of the support was due to a deepening cynicism about the Republic and its institutions, and some liberals like Ernst Lemmer joined the NSDAP outright. Other liberals, especially those influenced by Friedrich Naumann's thinking, disliked the crudeness of the NSDAP's politics and leaders, but saw some virtue in its economic and social policies. Hjalmar Schacht was the most prominent liberal in this category, seeing in dictatorial powers the key to restructuring the economy. Still other liberals saw the Enabling Act as the least-worst alternative between an Unrechtsstaat (lawless state) and a restricted Rechtsstaat (state defined by laws). By sanctioning the Act, the state was still acknowledging the importance of German law and jurisprudence instead of proceeding without the sanction of law. Finally, other liberals understood their affirmative vote as a tactical move that preserved their continued opposition to Hitler. A number of liberal recognized that the Act had enough support to pass, but also that Hitler was not above using extralegal means if thwarted. Therefore, it was important not to give Hitler a pretext to further clamp down on liberal politicians and intellectual circles. As the liberal leader Hermann Dietrich explained in a letter to a colleague angered by the vote:
after it was certain that all the bourgeois parties would vote for the Enabling Act, it became difficult for our representatives to do otherwise if they did not want to stand in opposition with the SPD. That would have had the same consequence that we would have been placed in the same camp as the Marxists, and it would have made it easier for government to take measures against us. It was also necessary, moreover, to take into consideration democratic officials, who have a difficult enough struggle already. Here again we had to support measures we privately opposed. This can no longer be avoided under the present circumstances.
In hindsight, this type of accommodation politics proved to be quite disastrous and only entrenched the dictatorship. Schacht, for example, was sidelined in the Economics ministry after his opposition to rearmament found little support in the Chancellery and far from reining in the NSDAP's reliance upon extralegal methods, the Enabling Act only encouraged these methods.
Much the same happened to the political Catholicism in Germany. From the Centre's perspective, the Act was a qualified win for the Church in that it still preserved the federal structure of the Reich and provided some sanction for Hitler's dictatorship by the elected representatives. This Catholic leadership certainly had memories of the Kulturkampf of Bismarck and feared that if they did not relent, then Hitler might pursue more extreme alternatives to the Act that would lead to civil war and possibly the victory of the KPD as the German right and middle tore themselves apart. But the Hitler state soon began to flaunt its agreements with the Church over issues like Catholic hospitals, schools, and exemption of seminary students from conscription.
The problem was the Enabling Act was -to paraphrase an aphorism of Talleyrand- it was worse than a crime against democracy, it was a mistake. Whatever agreements Hitler made with Kaas and the Centre leadership, the National Socialist state inevitably flaunted these agreements or violated them just as it did with Reichskonkordat. And the Third Reich soon emerged as the very paradigm of an Unrechtsstaat with more waves of arrests and arbitrary detention under protective custody following the Act's passage. By 1934, the bloom was off the National Socialist rose for many German Catholics and liberals- whether Catholic, Protestant, or agnostic- but they now had to live in a dictatorship they played a part in making.
Sources
Kurlander, Eric. Living with Hitler: Liberal Democrats in the Third Reich. New Haven, Conn: Yale University Press, 2009.
Langewiesche, Dieter. Liberalism in Germany. Princeton, N.J.: Princeton University Press, 1999.
McElligott, Anthony. Rethinking the Weimar Republic: Authority and Authoritarianism, 1916-1936. London: Bloomsbury, 2014.
Menke, Martin R. "Misunderstood Civic Duty: The Center Party and the Enabling Act." Journal of Church and State 51, no. 2 (2009): 236-264.
Peukert, Detlev. The Weimar Republic: the crisis of classical modernity. New York: Hill and Wang, 1992.