Some of the names being bandied for Mitt’s Veep are Catholics (Paul Ryan, Marco Rubio), but I think Mitt really needs to choose an Evangelical Protestant.
Mitt has done well among Catholics (better than Rick Santorum), and they will come out for him. The Evangelicals won’t vote for Obama, but some of them will stay home. Evangelicals have more of a problem with Mormons than Catholics do, so an Evangelical on the ticket would boost turnout for Mitt. Marco Rubio sometimes attends an Evangelical church, but he is thought of as Catholic.
If Mitt can’t turn out the Evangelical base in very high numbers, he’s in very deep trouble.