The state has a history of split-ticket voting, giving Heitkamp an opportunity to show her independence from national Democrats. She's publicly disagreed with Barack Obama on issues like energy, which she points to as evidence she won't be a rubber stamp. She enjoys high favorables and solid support from Republican ticket-splitters and has hammered Berg for his connection to a controversial real estate company.

With the lowest unemployment rate in the country, North Dakotans have their choice between two candidates with strong statewide appeal. This race will remain close until Election Day.

Ohio: Sen. Sherrod Brown (D) vs. Josh Mandel (R)

Amid the backdrop of what is arguably the nation's most competitive presidential battleground contest, first-term Senate incumbent Sherrod Brown hopes to prove that the Democratic wave that helped carry him into office in 2006 wasn't a fluke. Brown, who has a voting record the National Journal classified as one of the most liberal in the Senate, made a ripe target for Republicans looking for a pickup opportunity.

But in the home stretch of the 2012 campaign, Ohio remains conspicuously off the list of the most vulnerable Democratic-held seats. The Republican nominee is Josh Mandel, the 35-year-old state treasurer and Marine Corps veteran. Mandel has kept pace with Brown in terms of fundraising, but he continues to trail the incumbent by around 10 points in several independent polls from September and October.

Brown is far from having the race sewn up, especially with the daily volatility at the top of the ticket, but he has a leg up as the race enters its final days.

Pennsylvania: Sen. Bob Casey (D) vs. Tom Smith (R)

Recent polls have given Republicans hope where they previously had little. As Democratic Sen. Bob Casey's lead over Republican businessman Tom Smith began to tighten, the incumbent began actively campaigning across the state -- something he previously hadn't been doing.

Though Republicans have an opening now, Casey still enters the final stretch with the advantage. The state has not emerged as a contested presidential battleground and a strong showing by Barack Obama here could help Casey beat back this unexpected and late-breaking challenge.

Rhode Island: Sen. Sheldon Whitehouse (D) vs. Barry Hinckley (R)

Democrat Sheldon Whitehouse is a safe bet to win a second term in November. He faces Republican businessman Barry Hinckley.

Tennessee: Sen. Bob Corker (R) vs. Mark Clayton (D)

Unlike the tough battle he had in 2006 to take the seat, Republican Bob Corker should easily win a second term in November. He faces Democrat Mark Clayton, whose candidacy has been disavowed by the Tennessee Democratic Party for his involvement with an anti-gay rights group.

Texas: Ted Cruz (R) vs. Paul Sadler (D)

Open seat -- Sen. Kay Bailey Hutchison (R) is retiring

After a tough primary victory over the candidate endorsed by Gov. Rick Perry and the state party establishment, Republican nominee Ted Cruz, the former state solicitor general, is now heavily favored this November in his race against Democratic nominee Paul Sadler, a former state representative.

Utah: Sen. Orrin Hatch (R) vs. Scott Howell (D)

Republican Orrin Hatch learned an important lesson from his former colleague Sen. Bob Bennett in 2010 -- don't take the Republican nomination for granted. Hatch campaigned early and hard to win the GOP nomination over tea party favorite Dan Liljenquist, a former state senator. Hatch now is the overwhelming favorite to win a seventh term in November.

A Democrat hasn't represented Utah in the U.S. Senate in 36 years.

Vermont: Sen. Bernie Sanders (I) vs. John MacGovern (R)

Independent Bernie Sanders, the Senate's only self-described socialist, is expected to sail to a second term over Republican John MacGovern, a former Massachusetts state representative and unsuccessful congressional candidate.

Virgina: Former Gov. Tim Kaine (D) vs. former Gov./Sen. George Allen (R)

Open seat -- Sen. Jim Webb (D) is retiring

Republican George Allen is fighting hard to win back the seat he lost six years ago to now-retiring Democratic Sen. Jim Webb. A strong Democratic wave, along with a gaffe caught on tape that became infamous on YouTube, stripped Allen of what was expected to be his second term (and perhaps a launching pad to the 2008 GOP presidential nomination). He now faces Democratic nominee Tim Kaine, another former governor, in one of the marquee Senate races of 2012.

Polls remain close. Barack Obama's win in Virginia in 2008 and the strong play he's making for the state in 2012 have given Kaine organizational support. Allen has run ads linking Kaine, the chairman of the Democratic National Committee at the start of the Obama administration, to the president and to looming budget cuts. Kaine, in turn, has run ads tying Allen to George W. Bush's economic policies and that he's wrong on women's issues.