It turns out this year, especially, America's ticket-splitters could really hold the key. For Republicans to take over the Senate they might need to win a seat or two in a deep-blue Obama state, notably holding onto Massachusetts with Scott Brown or picking up Hawaii, where they're mounting a serious challenge. Democrats' fate, meanwhile, may rest on whether they can hold seats in red states that Romney will carry, like Montana, Missouri, or North Dakota.
These stand out, too, because these "split" states make up half of our top ten Senate states to watch. (The others races are in Presidential battlegrounds that could go either way and one, Maine, is a real wild card we'll get to later.)
This chart shows the possible split-ticket Senate races and who's favored in the presidential contest: