There are some countervailing considerations to your very good point. The evangelicals who might seriously object to Romney because he is a Mormon are located in safe states anyway. Second, the Catholics are preponderant in the rust belt states running West from Pennsylvania through Iowa and to a lesser degree into Michigan, Wisconsin and Minnesota. It might be that the Catholics in these battleground states will have more of a tipping effect than southern evangelicals in safe states. For the record, I'm not asserting that Iowa predominates with Catholics.
Finally, Catholics in the Southwest might just vote that way if Rubio were the choice not just because he is Hispanic but because he is Catholic.
As the polls move more and more to Romney's favor as I expect they will, we will have much more freedom to support a running mate on ideological rather than vote gaining grounds. Yes, the vice presidency is on paper an office without any power except to break a Senate tie. Some of the most powerful men in American political history have been neutered by the office, witness Lyndon Johnson. Yet other men like Dick Cheney, have infused the office with tremendous power by virtue of their personality and the trust and confidence reposed in Cheney by President Bush. Therefore, the right man in the right relationship with Pres. Romney could exercise decisive influence for conservatism.
We must not forget that, although the office carries with it virtually no power, it is a constitutional office meaning that the Vice President is literally independent of the president, does not serve at his pleasure, and cannot be fired. So it is quite conceivable that a Vice President could turn maverick and be terribly disruptive to the smooth running of any administration. That is why Romney could never pick a Newt Gingrich as his running mate. Romney will be looking for a team player not a bombthrower. Yet, the role of the Vice President who will be selected for his team loyalty must also be the bad cop on the campaign trail to Romney's good cop. Nixon played this role to perfection. Romney will be looking for a bombthrower on the campaign trail and a team player inside the White House.
If it looks like Romney is likely to win by the time of the announcement of the running mate, I would prefer a conservative who will go rogue if he thinks his oath to the Constitution demands it rather than a political choice who will carry a critical state or two. I am not above confessing that I agree with John Kennedy said, "first you gotta win" and at no time in my lifetime has that aphorism been more relevant than today. But if we are confident of the win, I want a man who we can be sure will regard the Constitution as superior to his duty of loyalty to his boss rather than a man who will be surefooted on the campaign trail.
A conservative vice presidential nominee will balance the ticket but will he balance the presidency? That depends almost entirely on the moral character of the man.
I agree that the role a VP will play in an administration will depend on the VP and, more importantly, on the president. Each of Mondale, Bush, Quayle, Gore and Cheney was considered by many to be “the most powerful VP ever” because the president gave them so much leeway for certain projects (and in Cheney’s case, because the president so valued his advice); we don’t hear that about Biden, though. So Romney picking a conservative runningmate would only make his administration more conservative if the VP and especially Romney want the VP to have an outsized role.
As for the importance of Romney picking a Protestant runningmate as opposed to a Catholic, I think that you underestimate the prevalence of Evangelical Protestants in many swing states. Evangelicals are a major presence in North Florida, rural VA and throughout NC, and Romney needs a big turnout among Evangelicals if he wants to carry the three must-win swing states of FL, VA and NC. Evangelicals are also a fairly large part of the electorate in OH and MI (but a bit less in PA), so failing to energize those voters would hurt Romney even in the Rust Belt. And, of course, Evangelical voters are a force in IA and CO, two swing states that could put Romney over the top.
Since its first presidential ticket in 1856, the GOP had two Protestants every time but one (1964, when the VP nominee was Catholic). It has never been an impediment for Catholics to vote Republican. The problem here is that Romney has both the problem of his past (and many fear, present) liberalism and the problem that he is a Mormon, so is not deemed to be a true Christian by many Protestant sects. (Or by the Roman Catholic Church, for that matter, but it isn’t as big a deal for Catholics.) So it would behoove Romney to pick a Protestant runningmate, with an Evangelical being even better. A Methodist such as Rob Portman would “hit the sweet spot,” since Evangelicals would recognize him as “one of them” while other Preotestants and Carholics would be perfectly comfortable with him. Throw in the fact that he’s from Ohio and is popular among blue-collar voters, plus his impressive résumé in both Congress and in the federal Executive Branch (two things missing from Romney’s résumé), and Portman is the ideal choice.
2012 should have been the year of the Catholic VP Republican, with Marco Rubio, Bob McDonnell, Susana Martinez, Paul Ryan, Bobby Jindal, Chris Christie, Pat Toomey, Rick Santorum, Newt Gingrich, Jeb Bush and Kelly Ayotte all making interesting VP picks for a Protestant presidential nominee. But we won’t have a Protestant heading our ticket, and I don’t think we can take the chance of not nominating a Protestant as VP.
The more conservative, the better, only NOT a SENATOR. We have to keep conservative senators in place!!