9 Senate races that are shaping up to be nail biters and will determine which party controls the Senate

Arizona congresswomen Martha McSally and Kyrsten Sinema are locked in a very close race to fill retiring Republican Sen. Jeff Flake's seat as the number of the undecided voters in the purple state has dropped.

A recent Fox News poll has both at 46% among lightly voters, and a recent CNN poll has Sinema up over McSally 51% to 47% — a lead that's within the poll's margin of error.

Incumbent Democratic Sen. Claire McCaskill is tied with Missouri's Republican attorney general, Josh Hawley, according to a recent Fox News poll.

Trump won the deeply red state by nearly 19 points in 2016, and, evidencing her desperation, McCaskill tried to distance herself from more progressive members of her party this week, insisting in a radio ad that she's "not one of those crazy Democrats."

Democratic Rep. Jacky Rosen is trying to unseat incumbent GOP Sen. Dean Heller.

A recent CNN poll has Rosen up by three percentage points over Heller. But Trump's approval rating as risen in the key swing state — to 49%, according to CNN's polling, among likely voters.

In a race on which the entire nation seems to be hyper focused, Democratic Rep. Beto O'Rourke is giving incumbent Sen. Ted Cruz, a 2016 presidential contender, a real run for his money.

A few indicators are pointing in the Democrat's direction: O'Rourke has raised more money than any Senate candidate in history — with no help from corporate PACs, voter turnout — particularly among millennials — is way up.

But Texas is a deeply red state and recent polls have Cruz ahead of O'Rourke. A new Emerson College poll released Thursday showed Cruz up by just three points.

Tennessee — a state Trump won by 26 points — is seeing an unexpectedly tight race to replace retiring GOP Sen. Bob Corker.

Centrist Democrat Phil Bredesen, the state's popular former two-term governor who also once served as mayor of Nashville, is running against conservative hard-liner Rep. Marsha Blackburn, who has Trump's strong endorsement.

A recent Fox News poll has Blackburn leading Bredesen by nine points among registered voters — a jump from the five-point lead she had in the same poll last month.

Read more: A Democratic campaign that looks nothing like the 'resistance' could be the Democrats' key to flipping the Senate

Incumbent Democratic Sen. Jon Tester is locked in a relatively tight battle with Republican Matt Rosendale, Montana's state auditor. A poll released by the nonpartisan group Gravis Marketing on Tuesday has Tester up by three points.

Tester has never won 50% of the vote in his red state, and Republicans hope the president's rallies — and his son Donald Trump Jr.'s multiple trips to Big Sky country — will help boost GOP enthusiasm.

Incumbent Democrat Bill Nelson has a slight two-point lead over Republican Gov. Rick Scott among likely voters, according to a Suffolk University/USA TODAY poll.

But Scott is suffering from low favorability in the Sunshine state, where 45% of likely voters disapprove of the job he's done as governor, while 43% approve.

Democratic Sen. Joe Donnelly currently holds the lead in his very competitive reelection contest in Indiana.

According to a recent NBC News/Marist poll, Donnelly is two points ahead of Republican Mike Braun, who has gained on the incumbent in a state Trump won by 19 points.