For the record, I totally reject your theory that there was an alliance. What existed is the same thing that exists today between the Catholic Church and the Republican Party: a shared moral belief. The Catholic Church sides with the Republican Party today on many moral issues, just as it does with the Democratic Party. If in the past there were more shared goals with the Democratic Party, that was a passing coincidence, not an alliance. The idea that the Bishop’s policy on immigration is based on ties to the Democratic Party more than the words of Jesus is illogical, even Chick-ish. It ignores the moral teachings of non-Americans, like Pope Benedict XVI in Caritas in Veritate.
I suggest you do a little research on Msgr. John Ryan. But before that we have the support that Catholics gave to Al Smith in 1924-28.
Take a look at the rise of the KKK after WWII, with its xenophobia, its antisemiticism, its negrophobia, and most significantly, its anti-Catholicism. It bitterly opposed the candidacy of Smith, as did the Bryan faction of the Democratic Party. In 1928, Smith got his turn, but in the face of the (ap arant) prosperity of the country, Hoover defeated him in a landslide. Smith lost several Southern states, the first time a Democrat has lost these states since the civil war. Smith represented the progressive wing of the Democratic Party in which the unions were prominent.
In 1932 FDR got the nomination with the support of a wide range of interests, including labor unions in which Catholics were deeply involved. Catholics had long been prominent in the urban machines. The machines, the unions, the South and–discordantly, the Communists (but not the Socialists) helped form the New Deal coalition. Father Ryan was a leading player in this alliance.
You know that Harry Truman came from the Pendergast machine. He had been the captain of a largely Catholic artillery company, and he got along famously with Catholics although he was a Baptist. In 1944, the VERY progressive (read-pinko) Henry Wallace was forced out of the vice-presidency by conservatives who did not like his pro-Soviet bent. Jimmy Byrnes, an ex-Catholic was blocked out by his divorce. Into this vaccuum Bob Hannegan the head of the National Committee pushed the nomination of Truman. Harry Truman was a strong New Dealer but the major clue was his undoubted anti-Communism
The Vietnam War opened a breech between Catholics and the radicals in the Democratic Party. Talk about coincidence: the rise of the McGoverites and Roe. V. Wade together left less and less room in the Party for observant Catholics. The Bernardin Catholics tried to minimize that breech but we finally end up with the exclusion of Bob Casey, a strong social justice AND pro-life Democrat from leadership in the party. Since 1992, abortion has become a party shibboleth. In my state, Texas, two recent Democrats might have won high office in the state, except that they could not get funding from the national party except they proclaim themselves pro-choice. In Texas today, except in Austin, that is a losing position. Texas has become too important for the party to write it off just to placate radical feminists like Pelosi.