The most-watched South Carolina race is in the 1st District, where second-term Republican Rep. Nancy Mace is defending her Lowcountry-based seat against a primary challenge from former state cabinet official Catherine Templeton. Mace sparked anger among some in the GOP last fall after she joined with seven other Republican House members to back the motion to vacate that ousted then-Speaker Kevin McCarthy. The move fit into her idiosyncratic track record as a onetime-ally of the old school GOP establishment who has evolved into an anti-establishment, Trump-minded conservative.

In fact, former President Donald Trump has endorsed her, a development Mace and her allies, such as Club for Growth Action, have emphasized in campaign ads. This reflects the incumbent's shifting positions, especially considering Trump endorsed Mace's primary challenger in 2022, whom Mace narrowly defeated. For her part, Templeton previously served in then-Gov. Nikki Haley's cabinet before mounting a failed gubernatorial bid in 2018. And while Mace has substantially outraised Templeton — $2.3 million to $663,000 as of May 22 — Templeton has received ample outside support. Groups have spent $5.4 million backing Templeton or attacking Mace, according to OpenSecrets, compared with $2.6 million supporting Mace or criticizing Templeton.

Notably, some pro-Templeton money may have come from sources allied to McCarthy, and Mace has denounced Templeton as McCarthy's "puppet." The McCarthy-aligned American Prosperity Alliance is running ads against Mace, and reporting by the Charleston Post and Courier found that the brand-new South Carolina Patriots PAC — which has spent $3.8 million to boost Templeton — received a small amount of money from the APA. SCP has run ads that argue Mace is weak on border security, while Templeton has played up her conservative business and immigration bona fides.

Limited public polling suggests Mace is more likely than not to survive. An early May survey by Kaplan Strategies found Mace leading Templeton 43 percent to 21 percent, while a late May poll from Emerson College/The Hill put Mace ahead 47 percent to 22 percent. But there's a small chance that this race could go to a June 25 runoff, as a third Republican candidate could win enough votes to keep Mace or Templeton from winning a majority.

—Geoffrey Skelley, 538

The second-biggest news of today here at 538 — after these primaries, natch — was the launch of our 2024 presidential forecast model. Our podcast host Galen Druke sat down with Editorial Director of Data Analytics G. Elliott Morris to talk about what goes into the model and what it says is the most likely outcome in November:

—Nathaniel Rakich, 538

Welcome to 538's live blog of the South Carolina, Maine, North Dakota and Nevada primaries! The major action tonight is all on the GOP side, where we're tracking competitive primaries for U.S. House seats, a Nevada Senate seat and North Dakota's governor's mansion. Here's a quick rundown of what to expect.

First polls close South Carolina at 7 p.m. Eastern, where tonight's GOP primaries are tantamount to election in three solidly red seats. Next up is Maine, where polls close at 8 p.m. Eastern and voters will decide which Republican will try to unseat Democratic incumbent Rep. Jared Golden in a red-leaning district. Polls will also close at 8 p.m. Eastern in most of North Dakota, where the GOP primaries will effectively determine the state's next governor and at-large U.S. representative.

Rounding out the evening, Nevada polls close at 10 p.m. Eastern. Republicans in the Silver State will choose their fighters to take on Democratic Sen. Jacky Rosen in the state's battleground Senate election, along with three House districts that they're hoping to flip from blue to red this fall.

Keep this page open for live updates and analysis on these races and others up and down the ballot!

—Tia Yang, 538

Tracking anti-abortion candidates again in tonight's primaries, I'm thinking about how some of these races will shape the issue this fall. In deep-red districts, Republican voters often choose from a slate of anti-abortion candidates. But many of their extreme positions are unpopular overall, and that could make a difference in competitive races. That may be why candidates that are less extreme on the issue are winning in primaries like Nevada's Senate race — Brown has championed a more moderate position, and his wife has opened up about an abortion she had when she was younger. Likewise, the anti-abortion candidate in Nevada's battleground 3rd District is far behind. The issue might not always rank at the top for voters in surveys, but the GOP seems willing to be a bit more moderate in races where an extreme anti-abortion candidate could turn voters off.

—Monica Potts, 538