Growing up in the 1960s I eventually heard the good guys, Americans, and in particular President Richard Nixon, called Nazi by leftist agitators. Although I was far better at math and science than at languages and human relations, in college I majored in Political Science because I did not like science being used to do evil, like dropping bombs on Vietnamese peasants. A considerable amount of my course work covered the Adolf Hitler's German National Socialist (Nazi) regime. Since that time I have read and even written a fair amount about the Nazis, trying to understand their particular brand of evil, and how it relates to evil in general. Most notably I have focused on how the National Socialist Party came to power. And I have related that to another major area of interest, the Catholic Church. I am the only modern writer I know of who emphasizes the fact that Adolf Hitler was born a Catholic, was a member of the Catholic Church in good standing throughout his lifetime, and died a Catholic [See Hitler's Catholicism]. Although factually true, this is considered heresy in the United States where the Catholic Church remains powerful enough within the Democratic Party and where that Party has dominated American society since 1932.
And yet today I learned something new and revealing. I will start with what I knew yesterday, that Adolf Hitler came to have dictatorial powers even though the National Socialists were not elected to hold a majority in the Reichstag (German equivalent of the U.S. House of Representatives). The Reichstag voted to give away its powers to the Chancellor, who happened to be Hitler. This happened on March 23, 1933.[Coincidently, President Franklin Roosevelt became President of the United States (later President for Life) on March 4, 1933.] You can find this in the most standard of works on Nazi Germany, The Rise and Fall of the Third Reich by William L. Shirer on page 199:
"Monsignor Kaas, the [Catholic Center Party] leader, had demanded a written promise from Hitler that he would respect the President's power of veto. But though promised before the voting, it was never given. Nevertheless the Center leader rose to announce that his party would vote for the bill. Bruening remained silent. The vote was soon taken: 441 for, and 84 (all Social Democrats) against. The Nazi deputies sprang to their feet shouting and stamping deliriously ..."
So the Enabling Act passed, and Adolf Hitler became dictator of Germany. Recall that this was in the midst of the Great Depression.
Given that you now know that Hitler was Catholic, and that the crucial votes that made him dictator came from the Catholic Center Party, you might think that you now understand what happened. But the Catholic Center Party was not actually fond of Hitler. There were more conservative, smaller Catholic parties in Germany that did like Hitler, but the Center Party actually believed in the democratic process. They were in their own struggle with the Vatican during this period. They were influenced by modern ideas. And a series of popes, up to then pope Pius XI, hated modern ideas, hated democracy, and in particular hated any grass roots control, or even bishop level control, of the Catholic Church.
Pius XI's man on the spot was Eugenio Pacelli, who would later become Pope Pius XII. He had been the Vatican's ambassador to Germany, or papal nuncio, from shortly after World War I until 1930, when he became Pius XI's Secretary of State. He had always been in charge of negotiating a treaty, or concordat, between the Roman Catholic Church and the German State. For the Popes the concordat was about imposing the relatively novel idea that the Pope had dictatorial powers within the Church, including appointing all bishops, but reaching down even to the parish level. Because Germany had been the center of the Holy Roman Empire, the Church in Germany was used to a great deal of autonomy, and still remembered when it had the right to appoint Popes.
Pacelli made a deal with Hitler on behalf of the Pope. Hitler would be allowed to become the political dictator of Germany, if Hitler would allow the Pope to become the spiritual dictator of Germany, or at least of Catholics in Germany (Hitler and the Pope planned to merge the Lutheran Church into the Catholic Church).
So the Pope, through Pacelli, ordered the Center Party to vote for the Nazi dictatorship. This would in effect dissolve the Center Party, which was something the Vatican had been seeking since at least 1920. The Center Party itself probably would not have signed the concordat without revisions, because it thought the church should have a more democratic internal structure than was being advocated by Pacelli and Pius XI.
The negotiation of the Nazi concordat is a major theme of John Cornwell's Hitler's Pope: The Secret History of Pius XII. The Enabling Act specifically is covered on pages 92 and 133-137.
Books cited (used copies of these are usually cheap):