But a new wildcard has entered the mix: Wisconsin -- a seat long thought to be a Democratic lock that has now become a lot more uncertain.
To win the Senate, Democrats need to pick up four seats if Hillary Clinton takes the White House; five if Donald Trump becomes the next president, since the vice president breaks a 50-50 tie. And no matter which party controls the chamber, the party in power will almost certainly have a very small majority -- a handful of seats, at most -- a recipe for gridlock in the new Congress.
GOP Sen. Mark Kirk was always a long shot, running in a blue state in a presidential year. And all year long, he's been running behind his Democratic foe, Rep. Tammy Duckworth. But when he took a dig at Duckworth's ethnic background at a debate last week, the wheels seemed to have fallen off his campaign. Kirk winning now would be seen as the biggest upset of the cycle.
Republican-held seat in Wisconsin. Verdict: Leans Democrat
Speaking of upsets, Republican Sen. Ron Johnson is trying to pull one off himself, taking out the man who had long been a heavy favorite in the race, former Democratic Sen. Russ Feingold.
Johnson has shifted his campaign tactics with a mix of positive ads about himself and attacks against Feingold, and seems to have caught Democrats off guard. Buoyed by an influx of outside spending, and Trump outspending Clinton on TV in the state, Johnson has suddenly found himself down by just 1 point, according to a new Marquette University poll. Still, both sides acknowledge that Feingold is the favorite heading into Election Day, especially given that no GOP candidate has won a Wisconsin Senate seat in a presidential year since 1980.
Senate Majority Leader Mitch McConnell's super PAC plans to invest $2.2 million in the state as well to boost Johnson, it announced Thursday.
Republican seat in Pennsylvania. Verdict: Leans Democrat
Polls over the last couple weeks have consistently shown Democrat Katie McGinty holding a slim lead, including a CNN-ORC survey from Wednesday reporting that Republican Sen. Pat Toomey is now down five points
. Toomey has been working the Philadelphia suburbs hard to appeal to moderate, middle-of-the-road voters -- namely women -- to help power him to a victory. But the national headwinds may end up being too tough for Toomey to overcome.
Republican-held seat in Indiana. Verdict: Toss-up
Former Sen. Evan Bayh had long been viewed as the heavy favorite when he made a surprise entrance into the race to win back his old seat against Republican Rep. Todd Young. But Bayh has been badly damaged by a series of stories about his post-Senate life, his inactive voter status and damaging revelations about meetings with donors and lobbyists during his time in the Senate. And now polls show the race in a dead heat, and Bayh stumbling to the finish line. Republicans are feeling bullish about Indiana.
Republican-held seat in Missouri. Verdict: Toss-up
Unlike in Indiana, Democrats are making the case that Republican Sen. Roy Blunt is the true Washington insider with deep ties to lobbyists, and has lost touch with his state, banking on the fresh-faced, 35-year-old Jason Kander to present himself as the outsider. Republicans privately say that Kander is perhaps the best Democratic recruit in the country -- and that Blunt has struggled to define him, prompting GOP fears that the veteran could cost them a critical seat. But Blunt is relying heavily on the fact that Missouri is still a red state and Trump is likely to win there, hoping to ride the GOP nominee's coattails to victory.
Republican-held seat in North Carolina. Verdict: Toss-up
Sen. Richard Burr's candidacy has been a constant cause of stress for GOP leaders. They don't think he has worked hard enough to define his Democratic opponent, Deborah Ross, or make the case that he should be reelected. More money has started to pour into the state to define Ross as an out-of-touch liberal elite, picking apart her record working as a top attorney for the North Carolina chapter of the liberal American Civil Liberties Union.
Burr is also struggling because Trump is underperforming against Clinton, who leads there. And new scrutiny over Burr's private comments at a fundraiser last weekend -- suggesting some gun owners may want to incite violence towards Clinton -- has put him on the defensive in the closing days of the campaign.
Democratic-held seat in Nevada. Verdict: Toss-up
Nevada's race has been remarkably tight for the duration of the campaign. Polls have either shown Republican Rep. Joe Heck with a lead within the margin of error, or shown Democrat Catherine Cortez Masto with a slight lead.
But Heck has had a rough few weeks in his handling of Trump, getting negative scrutiny from the right for his disavowal of the GOP nominee -- but also saying privately and publicly he's not sure for whom he would vote. Democrats are banking on Senate Minority Leader Harry Reid's political machine to help turn out Democratic voters. But the GOP believes Reid's unpopularity will be a stain on Masto's chances.
Republican seat in New Hampshire. Verdict: Toss-up
The race between Republican Sen. Kelly Ayotte and Democratic Gov. Maggie Hassan has swung back-and-forth for months, and Election Day is bound to be a true nail-biter. Ayotte had a rough October, with a poor debate performance where she called Trump a role model -- only to take it back -- and then being bombarded by an influx of Democratic money across the airwaves.
But Republicans have struck back, dropping major dollars across the airwaves in recent days, helping to fortify Ayotte's position, making the race the truest of toss-ups.
Republican seat in Florida. Verdict: Leans Republican
Sen. Marco Rubio heads into Election Day as a favorite against Democratic Rep. Patrick Murphy, buoyed by GOP money, name recognition and winning over Latino voters in Miami-Dade County. But Florida is the ultimate swing state -- and no statewide race there can be taken for granted. Murphy has benefited by a renewed push by the top of the ticket -- and by President Barack Obama -- hoping to ride Hillary Clinton's coattails into office, even though Democratic groups have mostly avoided spending in the expensive state.
A CNN-ORC poll out Wednesday showed Rubio clinging to a one-point lead, but most operatives on both sides believe the GOP senator is in a more comfortable position.
Republican seats unlikely to flip:
Ohio seat held by Rob Portman.
Arizona seat held by John McCain.
Iowa seat held by Chuck Grassley.