Making a long story short, it may come down to the race in Washington state between incumbent Dem Pat Murray and GOP challenger Dino Rossi.

For the rest, read this scenario by Chris Cillizza, where he gives the GOP easy wins in IN, ND, AR, and probable victories in WI and PA.

From then on, it gets a little dicey:

Polling in Colorado, Nevada, Illinois and West Virginia suggests each of those races is a genuine tossup; in a year in which the national winds are blowing strongly in Republicans' favor, it's not unreasonable to assume that GOP candidates in each will get just enough benefit from that breeze to win (a la Webb and Tester in 2006). That makes plus nine for Republicans - still one short of the majority.

The fourth tier: The majority then comes down to three states in which Democrats have heavy demographic advantages: California, Connecticut and Washington. (Barack Obama carried that trio by 24, 23 and 18 percentage points, respectively.) Connecticut appears to be the weakest opportunity of the three, with polls showing state Attorney General Dick Blumenthal (D) ahead of former World Wrestling Entertainment executive Linda McMahon (R) by double digits. And the cost of running for office in California may well keep Barbara Boxer (D) in the Senate. That means that the race in Washington between Sen. Patty Murray (D) and former state senator Dino Rossi (R) could be the linchpin on the narrow hopes Republicans hold out for control of the chamber.

Currently, Murray holds a narrow 6 point lead over Rossi. But considering the volatility of the electorate, that lead may be a mirage. If Rossi wins, however, he will have to make it a somewhat convincing victory given the shameless shenanigans pulled by Democrats in his race for governor in 2004 when he ended up on election night ahead (and won the first two recounts), but where the Democrats just kept counting and counting (and "finding" ballots in King County) until Christine Gregoire was declared the winner.

Just a gut feeling but I don't think the GOP will win all four races in CO, NV, IL, and WV. Illinois especially seems a longshot at this point. The Cook County machine is gearing up to give Giannoulis a huge majority in Chicago and near suburbs, while the national party is pouring money into the state to prevent the loss of Barack Obama's senate seat. And with only tepid support from conservatives, Mark Kirk would need to ride a downstate wave of massive disgust for the Democrats in order to make up the difference.

I think the outer reaches of success for the GOP at this point would be 8 seats - a good result by any measurement but a little short of a majority.