Senator Claire McCaskill is believed to be one of the most vulnerable Democrats in the Senate because of her low approval rating and Mr. Trump’s victory in Missouri by nearly 20 percentage points. Six years ago, she managed to win re-election largely because of the deep unpopularity of her Republican opponent, Todd Akin, who drew national ridicule and condemnation for claiming that women did not get pregnant in cases of “legitimate rape.” This time, Republicans are hoping to nominate a stronger challenger.

The front-runner is Josh Hawley, Missouri’s conservative attorney general, for whom Mr. Trump has raised funds and campaigned; his long-shot opponent is Austin Petersen, a libertarian. Polls show an extremely close race between Mr. Hawley and Ms. McCaskill.

In the First Congressional District, Cori Bush is running against an incumbent Democrat, William Lacy Clay, in a test of whether the progressive playbook pioneered by Alexandria Ocasio-Cortez in New York can work in the heartland. Read more about that race here.

Voters will also weigh in on a ballot measure that will determine whether Missouri becomes a right-to-work state, meaning that people would no longer have to join a union or pay fees if their workplaces are unionized. Republicans passed right-to-work legislation in 2017, but labor groups collected enough signatures to prevent the measure from taking effect pending a statewide ballot.

We’ll have live results here from Missouri.