Rob Portman is an acceptable running mate, but not a great one. On the plus size is that he helps in Ohio, has a middle-class background (but attended elite private schools as a child), and that conservatives have nothing in particular against him. But conservatives aren’t excited by him, either. Also, he’s tainted by having worked for the Bush Administration and is even more establishment than Romney. Also, remember that he’s only won one statewide election in Ohio so far, so he doesn’t guarantee it. I’d describe him as a middling choice.
The Protestant thing is not a big deal to most voters and I don’t think that a Mormom/Catholic Republican ticket will cause Evangelicals to support Obama.
It’s not about Evangelicals switching to Obama; it’s about them staying home. President Bush was reelected in 2004, despite increased Democrat turnout, largely because he got millions of Evangelicals who hadn’t voted in 2000 to vote that year. Taking the attitude of “what are they gonna do, vote for Obama?” is a good way to guarantee low conservative turnout and an Obama victory.
And it is pretty clear that (i) Romney can’t win if he doesn’t carry at least one of OH, PA or MI, (ii) Romney would likely lose to Obama in all three states if the elections were held today, and (iii) none of the trendy VP picks would deliver one of those states to Romney while helping him hold must-win VA, NC and FL (Romney will be on his own to pick up the one extra state he needs, be it NH or NV). While Portman has only won statewide in OH in a single election, that’s the same number of statewide wins as Rubio in FL or Martinez in NM, and one more than Ryan in WI, and Portman won handily in a year in which the gubernatorial election was very close. If Romney can’t carry OH without Portman on the ticket, then he won’t carry it with any of the other VP candidates and Obama will be reelected.