Who will win the Senate? These 9 races are the ones to watch.

With the battle royale of Donald Trump vs. Hillary Clinton dominating the attention of Americans, it's easy to ignore the other races and initiatives on the ballot on Nov. 8. But don't sleep on the Senate — control of which is being very hotly contested, and is critical to how America is governed over the next two years.

Republicans won the chamber with a majority of 54 seats to the Democrats' 46 (counting the two independents in the Democratic caucus) in the 2014 midterm election wave, coming at the sixth year of President Obama's term. Two years later, things are looking much better for Democrats, with Clinton the odds-on favorite to win the presidency. Still, on a seat-by-seat basis, the Senate is going to involve a lot of close calls. And those close calls would mean a huge difference for Clinton's potential ability to govern — most notably, in making appointments to the Supreme Court, lower courts, and many executive offices.

Of the 34 Senate seats up for election, exactly half, 17 each, are in states won by either President Obama or Mitt Romney in 2012. Furthermore, Republicans are defending 24 of these, compared to only 10 held by Democrats, due to the heavy GOP victories in the previous six-year cycles of 2004 and 2010. To win control of the Senate, Democrats must win at least four net seats — assuming that Clinton wins the presidential election, thus installing Tim Kaine as vice president to cast a tie-breaking vote.

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There also might be a real X-factor in these races: Whether the disunity in Republican ranks might hinder their traditional operations for getting out the vote. There is some preliminary evidence for this in states with early voting, with registered Democrats either racking up advantages in turnout, or narrowing the gap in those states where Republicans have usually voted early in greater numbers. If Democrats remain more energetic than Republicans, this could potentially tip the balance in some close races.

But which races? These are the nine to watch.

1. Nevada

This is the only state where Democrats are genuinely playing defense, with Senate Minority Leader Harry Reid's retirement. The state's Republicans swept to victory in the 2014 midterms, and polling this year has often shown a close race between Clinton and Trump. But that situation has also started to change, with Clinton pulling ahead, along with a noticed Democratic advantage in early voting. The Democratic nominee is former state Attorney General Catherine Cortez Masto, against Republican Rep. Joe Heck. Most polling this year has given Heck the lead, but a recent CNN poll had Cortez Masto up by 7 points, while Monmouth University had Heck still up by just 3 points.

From here, we get to a number of Republican-held seats, ranging from vulnerable spots to outside shots at Democratic upsets.

2. Illinois

This seat is the most clear case of a "gimme" for Democrats. First-term Republican Sen. Mark Kirk won narrowly in the 2010 GOP wave, and is now swimming against a very different tide. In June he refused to support Trump, but it doesn't seem to have done him any good in a state that is expected to go deep blue. A poll from early this month by Southern Illinois University showed the Democratic candidate Tammy Duckworth, a congresswoman and disabled Iraq War veteran, leading Kirk 48 percent to 34 percent.

3. Wisconsin

After Illinois, this is the next most obvious pickup for Democrats — though not quite as certain. Republican Sen. Ron Johnson is being challenged in a rematch by Democratic former Sen. Russ Feingold, whom Johnson defeated in 2010. Feingold has led by substantial margins in most polls ever since this race began, but earlier this month it appeared to be tightening; Marquette University's poll had Feingold up by only two points. Since then, however, polls from Monmouth University and St. Norbert College have had Feingold up by 8 and 12 points, respectively. This is a state where Clinton is likely to win by a solid margin, so Johnson will need a lot of ticket-splitters in order to hang on.

4. Indiana

In the home state of Gov. Mike Pence, Republican Sen. Dan Coats is retiring. Democrats recruited former Sen. Evan Bayh, who had retired in 2010, to come back and run once more against the Republican nominee, Rep. Todd Young. At first, this looked like a shoo-in for Bayh, but things have narrowed after Young assailed Bayh as a consummate D.C. insider. Indeed, some are already writing of Bayh's "collapse." But Bayh remains ahead, with a recent poll by Ball State University putting the Democrat up 6 points. As with so many other states, a lot depends on the margins in the presidential race here.

5. New Hampshire

In another swing state likely to vote for Clinton, Republican Sen. Kelly Ayotte is being challenged by Democratic Gov. Maggie Hassan. In August, Hassan blundered when she initially dodged questions about whether Clinton is "honest and trustworthy" — but that has since been more than overshadowed by Ayotte's own mega-gaffe, when she said at a debate early this month that Trump was a "role model" for children. This was just before the tapes emerged of Trump making casual comments about sexual assaulting women. Ayotte now says that was a mistake, and is trying to run as a check and balance on an all-but certain President Hillary Clinton. Polls on this race are a mixed bag, ranging from a 9-point Hassan lead to a 3-point Ayotte edge.

6. Pennsylvania

Republican Sen. Pat Toomey is being challenged by former state Environmental Protection secretary Katie McGinty. At their final debate, held on Monday night, Toomey insisted that whether or not he supports Trump really isn't an issue: "I don't think my constituents care that much how one person is going to vote — they're going to make their own decision… I think they care much more about whether I've got policies that are going to grow this economy." A recent Quinnipiac University poll showed Toomey ahead by 4 points.

7. Florida

It's been a strange year for Marco Rubio. He lost the Republican nomination for president to Donald Trump — culminating in a humiliating landslide defeat in his home state's Republican primary. Rubio had previously said he would not run for another term in the Senate — but he was eventually wooed back into running. When asked at a debate this month whether he would run for president again, Rubio answered: "I'm going to serve six years in the United States Senate, God willing, and I'm looking forward to it." His Democratic opponent is Rep. Patrick Murphy, who was reportedly abandoned by the Democratic Senatorial Campaign Committee last week, after polls had consistently shown Rubio ahead. But the latest poll results are now very close indeed: CBS News had Rubio up by just 2 points, and a local Fox affiliate had them tied at 46 percent each.

8. North Carolina

This swing state voted narrowly for Obama in 2008, then narrowly for Romney in 2012 — and this year, Clinton has been edging ahead of Trump. Two-term Republican Sen. Richard Burr is being challenged by former state Rep. Deborah Ross, who sought at their one and only debate to tie Burr to Trump. A recent Monmouth poll had Burr up by six points, while a Siena College poll released Tuesday morning had the Democrat pulling ahead by a 1-point margin.

9. Missouri

This one is a dark horse. In a state that has become reliably Republican for the presidential race, incumbent Republican Sen. Roy Blunt is being challenged by Democratic candidate Jason Kander, Missouri's secretary of state and a veteran of Afghanistan — who is turning the political insider charge against Blunt — a theme that might have some teeth in a year when Republican voters nominated Trump. Kander also had a memorable ad in which he demonstrated his knowledge of guns, as well as his support for both gun rights and background checks, by assembling a rifle while blindfolded. A recent Emerson College poll had them tied at 44 percent each.