Here's a look at the major battlefields:
Looking Good for Democrats
1. Ohio: Senator Sherrod Brown (D) vs. Josh Mandel (R): Early this year, the freshfaced Mandel (he commonly makes jokes on the stump about hoping to shave someday) was touted as the next Marco Rubio -- a young, charismatic Republican who was headed for the Senate. But Mandel has struggled, weighed down by a seeming lack of knowledge on many policy details, questions about his ethics both as state treasurer and as candidate, and his own gaffe-prone demeanor. The race has also seen high outside spending on both sides. Nonetheless, Brown has maintained a lead -- never huge, but consistent -- and seems poised to win a second term.
2. Massachusetts: Senator Scott Brown (R) vs. Elizabeth Warren (D):
Although it took her some time to find her footing -- especially given early stumbles over her debunked Native American heritage -- the populist hero and Harvard professor seems to have gotten into a groove. Brown is an incumbent (though not a regular one, as he's only served two years of Ted Kennedy's unexpired term) and has sensibly run to the center. But he faces the same challenging landscape of any Republican running the nation's bluest state. Most polls have given Warren a solid lead in the upper single digits -- although a Boston Globe poll released Monday may give her pause, as it shows the race tied. The pair's final debate, scheduled for Tuesday, was canceled due to Hurricane Sandy.
3. Virginia: Tim Kaine (D) vs. George Allen (R):
In the face-off between two former governors, Kaine seems to be have the edge on Allen, who is trying to recapture the seat he lost to Senator Jim Webb six years ago; Webb is retiring after just one term. Interestingly, Kaine has tied himself to the president, whose fortunes in the state look dimmer than they did a month or two ago. Presidential coattails could be a deciding factor in the race.
4. Connecticut: Chris Murphy (D) vs. Linda McMahon (R):
On her second attempt at a Senate seat in two years, McMahon -- former CEO of World Wrestling Entertainment -- has outspent Murphy, a congressman, five to one, and the polls show she's closer than she was in her race against Senator Richard Blumenthal. But she still trails in every poll, and is unlikely to reverse that trend before Election Day, especially in a Democratic state. Senator Joe Lieberman, an independent who caucuses with Democrats, is retiring.
5. Pennsylvania: Senator Bob Casey (D) vs. Tom Smith (R):
This could be a sleeper race. Pennsylvania has been reliably, if moderately, Democratic territory for some time, and the incumbent Casey was thought to be safe. But a few polls in late October have seen his lead fall to single digits. Meanwhile, Mitt Romney is launching a new push to expand the presidential map into the Keystone State. While still a longshot, Romney's coattails could help Smith to a surprise win.
Down to the Wire
1. Missouri: Senator Claire McCaskill (D) vs. Todd Akin (R):
This should have been an easy pick-up for the GOP. Akin, a U.S. representative, managed to run an impressive insurgent campaign and win the primary -- then exploded his candidacy with his comments about "legitimate rape" and conception. Although most leaders in his party promptly jettisoned him, some members have come back around. No poll has shown Akin leading since his comments, but McCaskill remains a very weak incumbent closely tied to Obama in a state that really doesn't like the president.