Some clarifications here are appropriate.
First, the “Catholic vote” needs to be further defined. Does that mean regular Mass-going Catholics or people who identify themselves as Catholics merely because they were baptized as Catholics decades ago and might occasionally go to Mass on Christmas and Easter? Does “Catholic vote” include people such as John Kerry and Teddy Kennedy who are active church members but are in open and public disagreement with the church?
Using the term “Catholic vote” without some breakdown of what that means is about as useful as saying “Protestant vote” without clarifying that to indicate whether the “Protestant vote” means “active churchgoing Protestants” or “Protestant evangelicals.”
Second, Santorum had no choice but to interact with Kennedy's speech. What JFK said years ago was the major modern declaration by Roman Catholic liberals that they would not allow their personal faith to control their policy decisions. That speech is a key part of why modern Roman Catholic politicians feel free -- much like the Mormon Mitt Romney did as governor of Massachusetts -- to say that they're personally opposed to abortion but won't impose their religious preferences on people who are not members of their church. Santorum had no choice but to give his response to that speech, either by trying to defend it, trying to clarify it, or doing what he actually did of attacking the speech head-on. You may not like what Santorum said, but his comments are in line with what the Roman Catholic Church says about politicians denying in their policies what they profess to believe.
Third, all candidates make mistakes in their speeches. Vice President Joe Biden has a reputation for off-the-cuff comments that aren't terribly helpful to his campaign. Former President George W. Bush said such things too. Where Santorum is drawing fire is on different issues. It appears that Santorum is not getting in trouble for mis-speaks but rather for comments that he actually believes — and in many cases, things on which many or sometimes even most social conservatives agree with him.
You may say “Who cares if two college kids use a condom on Sat. night.” For a lot of us, unless those two college kids are married, we care a great deal. Rampant and rapidly increasing sexual promiscuity is not something social conservatives consider a minor issue. You may disagree, but please understand that for a lot of Republicans we definitely **DO** care about the breakdown of the family and sexual morality in America.
That issue of personal morality is a core part of why lots of evangelicals are unhappy with Newt Gingrich. I'm willing to vote for Gingrich if I have to do so though I don't believe he's the best candidate. I've worked hard in my own circles to present Gingrich as an acceptable candidate. I've circulated a letter by a San Diego pastor regarding Gingrich's personal faith which gives me a fair amount of reassurance that Gingrich in fact **HAS** repented of his past adultery. Unfortunately, a lot of conservatives don't buy it, and I'm deeply concerned that we're going to end up with Romney because many social conservatives can't stomach Gingrich's past immoral behavior and many economic conservatives can't stomach Santorum’s emphasis on personal morality.
Getting Romney as the Republican nominee is close to being the worst of all possible worlds.