President Barack Obama's lead in the polls is increasing. And so is people's dislike of Mitt Romney, even by people who say they'll vote for him.
So, the conservative-moderate Republican candidate for president sees his chances for winning the election slipping, and he goes for a Hail Mary pass to shake things up (or at least mobilize the ultraconservative base). Been there; done that. Remember John McCain and Sarah Palin?
Romney's selection of Wisconsin's Rep. Paul Ryan for his vice presidential candidate is an admission of fear. You don't take a risk and pick a tea partyer like Ryan unless you think your chances of winning are poor.