You seem to be confusing Catholics with Jewish voters. If it "has always gone Democrat", how the heck did George W. Bush, Ronald Reagan, Richard Nixon, and Dwight Eisenhower WIN the Catholic vote when they were running for President? None of them were Catholic candidates.
That seems to reflect that Catholics vote how the nation as a WHOLE votes (unlike Jewish voters, who reflectively vote Dem, even if a Republican Presidential candidate wins the nation as a whole easily) John Kerry is a Democrat, and he LOST the Catholic vote, despite BEING a Catholic. So what happened to Catholics "always voting Democrat" in those elections?
Most pollsters disagree with your premise that Catholics are a safe Democrat voting bloc. That might have been true in the 1930s, but it certainly isn't true given how Republican Presidential candidates have won Catholics numerous times in recent decades. Jews are a safe Democrat voting bloc that has overwhelmingly supported every Dem Presidential candidate for at least 80+ years (in contrary to numerous Republicans who fantasize about how Jews are "in play" every election cycle), whereas Mormons are a safe GOP voting bloc that reliably votes Republican even when Republicans do poorly nationally (Bob Dole, John McCain, etc.). Catholics are as swing vote that tend to support the candidate that the country as a whole supports, which the occasional exception.
In fact, 2012 was the third straight Presidential election where the Catholic vote has been a near-carbon copy of the overall vote nationwide. If Catholics are "safely Democrat", shouldn't have Obama to win over much more than 50% of them? Shouldn't have John Kerry, a Catholic and a Democrat, won the Catholic vote in 2004 INSTEAD of George W. Bush?
That is the rare exception, many think they voted republican for the first time for Eisenhower’s “reelection”, having voted against him first (there is dispute over that), then the same for Nixon, for “reelection”, not in 1968, then Reagan, Reagan, and Reagan’s veep, and then George W’s “reelection” after having voted for Gore the first time.
During those years, the Protestant vote had rare exceptions as well, having gone democrat in 1932, 1936, and 1964.
There is no mystery to how Catholics vote, it is what the left depends on for their future, and what JFK had in mind as he worked tirelessly to change the immigration laws.