Ironically, voters in the Senate Majority Leaders native NV can vote a "nobody" option, and both he and his Scientologist Tea Party opponent are so awful there's been speculation a lot of people will. That's hardly the norm though; most states it'd probably just be tallied as an under or overvote if at all, then discarded. It's up to the states mostly though. The Constitution actually has a section that covers running elections:


It's up to each state, and each one has its own laws, which usually get enforced and modified by county and municipal laws. 2010 is actually an important year, because while each state always has 2 Senators, with the census it's time to divide 435 House seats by 310 million people, and each states legislature will do that with however many House seats that gives them by federal law. Texas, for example, is expected to get 3 more House seats, and it was already second behind CA with 34. Interestingly, we're the only state growing that much; CA is expected to stay at 53 after skyrocketing for years, and FL will go from 25 to 26 or 27. The Rust Belt will cost OH 2 more and drop them to 16, plus eight other states lose one and six others gain. That's important both because it's how big each states chunk of the new Congress is, and because state House+Senate seats=Electors in the 2012 Presidential election.

Each states Legislature will then divide up the map into however many Congressional Districts that creates. While they're at it, they'll draw up State Senate and House districts for however many seats they want the next state Legislature to have. Then the cities and counties will draw in their districts (with the same federal census numbers) and each level of government will set dates for its elections, with one big election every second year on the first Tuesday after the first Monday in November, as fixed by federal law (but not the Constitution).

Incidentally, this means the big plums for 2012 Presidential candidates will be:

CA 55
TX 37
NY 30
FL 29/30
PA 20
IL 20
OH 18
GA 16
NC 16
MI 16

Of those, really only FL, PA, OH and MI are likely to be in play, maybe NC if there's a strong black turnout and low general turnout. The Dems will get CA, NY and Obamas native IL, and probably MI, so that's 121. The Republicans will get TX and GA for sure, and probably OH and NC for 68. After that there's about five states with a dozen Electoral votes, which they'll likely split 2/3, maybe five more with about 10 that'll split much the same, and then it's 30 small state where the Dems need 100 EVs and the Republicans need 150. Trouble is, most of those states are either in the South or the West, and with a few exceptions like NM most of them are deep red. Since the Clinton era Dems have had to be strong enough in FL, OH or PA to win 2 of those 3, but that was when OH and PA had >20 votes and FL only had a little more. Now the big prize is FL, because it'll have almost 30, so it's the biggest state in play, then a big drop to PA and OH. If the Dems can get that and PA that's 50 more EVs and they only need 99 to win.

It is kinda sad though to hope that the Senate Majority Leader loses to a Scientologist just so the guaranteed Democratic Senate picks a firebrand Majority Leader. How far we've come in two years of Obama; now the only reason his seat, the VPs seat AND the Senate Majority Leaders won't go Republican is Christine O'Donnell running for Bidens old job. That's three SOLID Dem Senators gone, plus he pulled three more into Cabinet positions and now those are going Republican. If you really want to know what happened, look there.