Now that the Supreme Court has upheld the constitutionality of President Obama's health-care law, conservatives are paying close attention to the composition of the Senate, knowing that their last chance to overturn the law is with a Republican president and a Senate majority.

But looking at the Senate landscape, the odds of Republicans taking back the upper chamber are no better than 50-50, even with more Democratic seats in play, a favorable political environment, and an energized GOP base. It's not the often-maligned Mitt Romney campaign that's going to drag down the ticket; it's several of the candidates themselves.