Here are the states to watch.

Arizona

The Valentine State has not elected a Democrat to the Senate since 1988. Representative Kyrsten Sinema may be poised to change that. The third-term congresswoman was once a spokeswoman for the Green Party, but has sought to position herself as a centrist as she vies for Jeff Flake’s seat. She’s voted in step with President Trump more than half the time in the past two years, and apart from former Vice President Joe Biden, she has declined to campaign with nationally popular Democrats.

Read: Democrats want to flip six seats in California.

It’s a good strategy in an election where the popular Republican governor, Doug Ducey, is at the top of the ticket. If early-voting numbers are any indication—especially the surge in new voters and Millennial voters—Sinema’s challenger, Republican Representative Martha McSally, may have to rely on Ducey’s coattails to eke out a victory. The 52-year-old Air Force veteran is in many ways the Platonic ideal of a Republican candidate. But Sinema has managed to inch McSally into a corner for her support of the GOP’s attempt to repeal Obamacare, which would have stripped coverage for preexisting conditions.

A RealClearPolitics polling average gives McSally a 0.2-point lead over Sinema. “If Sinema can win, it would signify that Arizona is a purple state,” Representative Ruben Gallego of Arizona told me. “It would signify that with the proper investments, we can be a state that’s competitive.”

Nevada

Early voting in Nevada this year has outpaced that of the previous midterm election by 113 percent—more than in any other state. And the results don’t look good for the Republican incumbent Dean Heller. The key data point? Democrats are clocking a nearly 2,000-vote lead in Washoe County. That’s not to say Heller won’t ultimately win Washoe. But if these early-voting trends continue and Heller can’t clinch victory in the county that secured his win in 2012, it’s hard to see how he makes up the loss elsewhere. “The math just becomes very difficult for him,” the veteran Nevada political analyst Jon Ralston told me.

Heller is the only GOP senator up for reelection in a state won by Hillary Clinton in 2016. He’s contesting Jacky Rosen, a Democratic representative whom the opposition has dubbed “Wacky Jacky.” Like Sinema, Rosen has centered much of her campaign on health care, criticizing Heller for his support of repealing Obamacare. But unlike Sinema, Rosen has eagerly embraced the local and national party machine, promoting endorsements from former Senate Majority Leader Harry Reid, former President Barack Obama, and Senator Kamala Harris of California.

An Emerson poll released Monday shows Rosen with a four-point lead over Heller. “But the thing about Dean Heller is he’s never lost a race,” Ralston was careful to note. “So that should be in the back of everyone’s mind.”