The Current State of the US Senate
Currently, Democrats (including 2 Independents) have a slim 53-47 lead in the US Senate, meaning the Republicans only need to pickup a net of 4 seats for control of the chamber. If Mitt Romney and Paul Ryan win the presidential election, they would only need to net three seats as the vice-president acts as a tiebreaker for the US Senate. The Democrats have a disproportionate number of seats to defend as the current crop of senate members won in 2006, a heavily Democratic year. The momentum, however, has slipped dramatically since then and tilted heavily toward conservatives and Republicans.The Democrats Uphill Climb
With a whopping 23 of the 33 seats up for grabs, the Democrats are in trouble as they must attempt to defend a number of seats held by moderate party members who have cast unpopular votes in support of the Obama agenda. A Number of incumbent senators are retiring, making the seats even more vulnerable. Of seats not up for re-election this year the Republicans hold a 37-30 lead. Overall, we identified 12 currently-held Democratic seats that are probably safe for the Democrats and another 11 seats that range from being somewhat in trouble to probable losses. The Republicans would need to pick-up between five and seven seats and they will likely be favored to take Wisconsin, Nebraska, Missouri, and North Dakota and they have at least break-even shots in Virginia and Montana. Florida, Ohio, Connecticut, Hawaii, and New Mexico will all also feature expensive, high-profile campaigns where the Republicans will be underdogs, but have decent shots at pulling the upset.Republicans Must Minimize Their Own Losses
The GOP is in much better shape as they only have 10 seats to defend this year. We listed 5 that are at least moderately in trouble, but even three of those are likely to stay with the Republicans. The GOP is almost certain to lose Maine, while Massachusetts may be the purest toss-up the GOP faces. US Senator Scott Brown remains popular in the state - especially for a Republican - and he receives a decent amount of cross-over support from Obama voters. Having former Massachusetts governor Mitt Romney as the presidential nominee may even help turn out a few more down-ballot voters for Republicans than normal.Tea Party and Romney Combine for 1-2 Punch
While the tea party and Mitt Romney appeared to be early adversaries, the two sides have seemingly come together following his primary victory and the result could be a tremendous positive for both. While the tea party grassroots has remained galvanized, their support of Romney campaign has picked up dramatically over the weeks since his victory, and especially after Romney tapped Paul Ryan to be his running-mate. Romney, meanwhile, has been able to attract many independents and moderates who are at least willing to give the GOP a good look. Also, Romney's fundraising will lead to increased voter turnout efforts and, likely, greater down-ballot support.The 1-2 punch of a grassroots tea party movement and the more calculating but overly-efficient campaign stylings of the Romney campaign could provide an enormous voter boost in these competitive states. Meanwhile, it's possible that the Obama campaign will all but ignore a number of the more conservative states he expects to lose anyway, leaving the get-out-the-vote efforts and campaign visits behind and the incumbent Democrats to fend for themselves.

