What to watch: 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 18 percentage points. Six years ago, she managed to win re-election largely because of the deep unpopularity of her Republican opponent, Todd Akin. This time, Republicans are hoping to nominate a stronger challenger. Read more

The front-runner is Josh Hawley, Missouri’s conservative attorney general, whom Mr. Trump has fund-raised and campaigned for. Mr. Hawley’s long-shot opponent is Austin Petersen, a libertarian.

— MAGGIE ASTOR

Read more about today’s most important races.