President Trump is heading to Missouri as he pushes for Republican Josh Hawley to unseat Democratic Sen. Claire McCaskill in a critical Senate race. Mr. Trump's campaign on Friday announced the planned Sept. 21 rally in Springfield, Missouri.

McCaskill is a top target for Republicans seeking to expand the party's slim 51-49 edge in the Senate. She is among 10 Senate Democrats up for re-election this year in states that Mr. Trump won. She's considered among the most vulnerable incumbents as she faces Hawley, the state's attorney general.

Mr. Trump won Missouri by 18 percentage points in 2016.

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Hawley and McCaskill met for their first debate on Friday. The two spent much of the debate attacking each other, with Hawley finishing several answers by calling McCaskill a "Washington liberal" out of touch with Missouri values.

Both candidates emphasized their relationships with Mr. Trump, with Hawley embracing Trump talking points on immigration and other issues, and McCaskill trying to assert her independence from both the president and the Democrats.

Both candidates have raised millions of dollars for their campaigns.

Mr. Trump plans a heavy schedule of campaigning and fundraising through the Nov. 6 midterm elections. He cancelled a planned Thursday trip to Missouri in light of Hurricane Florence.

Mr. Trump has already traveled to states with vulnerable Democratic senators to rally on behalf of their Republican opponents. He recently visited West Virginia to support Patrick Morrisey and Montana to support Matt Rosendale.