He exudes a friendly enthusiasm, but the timbre of his voice hardens when he talks about “liberals” or "elites.”

“I’m not happy that people in Washington, D.C. — and, let’s be honest, New York, on Wall Street, in Hollywood — look down on the kind of upbringing I had,” Mr. Hawley said.

Mr. Hawley wants Missouri voters to think Ms. McCaskill looks down on them, too. He says she has “sold out the whole Midwest way of life,” a narrative that Ms. McCaskill — who grew up in a small town and waited tables to pay her way through the University of Missouri — rejects.

Meira Bernstein, her spokeswoman, said it was “absurd” to claim that Ms. McCaskill had lost touch with her constituents, including Trump supporters. She said the senator had held town-hall meetings across the state and had voted for about half of Mr. Trump’s cabinet nominees and most of his judicial nominees.

But Ms. McCaskill has made missteps in recent weeks that have highlighted her affluence and helped give traction to Mr. Hawley’s attacks.

Her financial disclosure forms revealed that her husband made between $230,000 and $2.1 million from a $1 million investment in a hedge fund connected to the Cayman Islands, an offshore tax haven, according to McClatchy. Ms. Bernstein declined to comment on the report.

And after announcing a bus tour of the state, Ms. McCaskill used a private plane to travel to some of the stops. Her spokeswoman said she paid for the plane trips herself, but the Hawley campaign has savored the optics of Ms. McCaskill flying from town to town across a state that winces at the term “flyover country.”