Missouri Senate: Until August of this year, Claire McCaskill, Missouri's incumbent Democratic senator, was considered one of the most vulnerable Senate Democrats up for re-election. Elected by a razor-thin edge in 2006 in a state that has since grown increasingly conservative, Republicans were counting on picking up a Senate seat in Missouri. But after a protracted primary fight yielded the ultra-conservative Rep. Todd Akin as the Republican nominee, the GOP has faced some set-backs in the state. Chief among those was Akin's August comments suggesting, inaccurately, that pregnancies don't result from "legitimate" rapes. Since then, McCaskill appears to be leading in the polls, and Akin has struggled to stay competitive.

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Arizona Senate: The battle to replace outgoing Republican Senator Jon Kyl, Arizona's junior Senator, has ramped up in recent weeks, with former surgeon general Richard Carmona edging in on Republican Rep. Jeff Flake. Flake entered the race as a strong favorite, but a recent infusion of outside cash seems to have helped give Carmona a boost.

Arizona - U.S. House, 9th District: Democrat Krysten Sinema v. Republican Vernon Parker: In one of Arizona's new congressional seats that includes parts of Phoenix and Tempe, the race between Sinema, who is openly bisexual with a politically radical past, and Parker, an African-American up-and-coming Republican, is one of the most watched Arizona House races. Each candidate is labeling the other as extreme and attacking at every corner.

Colorado - U.S. House, 6th District: Republican Rep. Mike Coffman v. Joe Miklosi: Rep. Mike Coffman is the incumbent facing state representative Joe Miklosi in the suburban district that stretches from Denver to Aurora, Colo. The newly remapped district includes a larger percentage of Democrats. Coffman's Tea Party principles are being challenged as he's trying to focus the race away from social issues, including abortion, which continues to plague his candidacy.

Minnesota - U.S. House, 6th District: Republican Rep. Michele Bachmann v. Democrat Jim Graves: Tea Party star Michele Bachmann spent much of 2011 on the presidential campaign trail. Now she has a formidable challenger in Jim Graves. But her fundraising heft is incomparable. She has raised at least $20 million dollars and spent most of it trying to hold on to her seat, according to the Center for Responsive Politics. Graves is running as a moderate and framing Bachmann as divisive.

Nebraska Senate: After Nebraska's moderate Democratic Senator Ben Nelson announced last year he wouldn't seek re-election, his party's prospects of holding onto the seat looked bleak. Even Democrat Bob Kerrey, a former senator and governor and well-known political entity in the state, doesn't seem to have much hope of winning a state that is now solidly Republican. Despite some recent tightening of the race, Nebraska state senator Deb Fischer appears to have the lead there.

Texas Senate: Tonight's Texas Senate contest isn't expected to be a close one: The Lone Star State isn't a battleground territory (though President Obama predicts it will be soon), and Republican Ted Cruz is expected to defeat Democrat Paul Sadler handily. But Tuesday will mark the culminating point of Cruz's unexpected rise: The Tea Party-backed candidate has no history in elected office, and is being touted as one of the conservative movement's rising stars.