I think in this article Cardinal Francis George of Chicago hits on why most bishops are so reluctant to publicly criticize Democrat politicians (for the most part) who are pro-abortion. Because deep down most bishops still want to vote Democrat.
“The great scandal, to my mind, of the Democratic Party is that there’s no pro-life caucus,” George told the crowd at the luncheon where he was guest speaker. “A party that historically has been concerned about the weakest among us—why, most Catholics were Democrats historically—doesn’t permit any freedom of speech around the question of abortion.
“One can say, as I have, that the Democratic Party has lost its soul,” he said, as the largely Democratic audience, which included Cook County Board President John Stroger, sat gaping back at him. Then, with the comic timing of a late-night talk-show host, the cardinal quickly added, “One could also argue that the Republican Party never had a soul,” as the audience erupted into belly laughs and applause.