In the final stretch of a tight Senate race in increasingly red Missouri, Sen. Claire McCaskill turned against her own party in a new campaign radio ad that declares she is “not one of those crazy Democrats.”

The unusual message has been hitting the airwaves in central Missouri for about a week, and it debuted as Republican challenger Josh Hawley’s internal campaign polls showed him taking a 7-percentage-point lead in a race that for months has been a dead heat.

The radio spot, which was first reported by CNN, features voices of two middle-aged men discussing the race. They take a few shots at Mr. Hawley for being a “man in a hurry” and spending to much time at the gym before turning their attention to the two-term incumbent.

“I don’t always agree with Claire McCaskill but she works hard, fighting against those tariffs, doing all those town halls,” the first man says. “Claire’s not afraid to stand up against her own party.”

The second man interjects: “Yep and Claire’s not one of those crazy Democrats. She works right in the middle and finds compromise.”

The McCaskill campaign refused to answer questions from The Washington Times about whom the candidate considers to be “crazy Democrats.”

Recent polls by news organizations still show the race in a virtual tie, but the national Republican Party is touting the internal polling.

Mr. Hawley, the state attorney general, has hammered the incumbent senator for being beholden the Democratic Party and doing the bidding of Minority Leaders Nancy Pelosi in the House and Charles E. Schumer in the Senate, while ignoring the will of Missourians.

He highlights her opposition to President Trump’s Supreme Court picks and her support of gun control and sanctuary cities.

Ms. McCaskill has cast herself as an “independent voice” and a “bipartisan dealmaker” throughout the campaign in a state Mr. Trump won by nearly 19 points in 2016, rendering her one of the most vulnerable Democrats this cycle and putting Missouri on the front line in the battle for control of the Senate.

Now she has amped up that message for the closing days of the campaign.

Democrat strategist Brad Bannon said Ms. McCaskill played it smart by targeting the radio at Republicans and independents in deep-red rural Missouri.

“You can target a message with radio. Running this ad in the St. Louis market with a concentration of Democrats would be a mistake,” he said. “It’s time to galvanize the base. Running the ad in rural areas would be a good way to rally independent voters.”

In the final debate of the race Thursday, Ms. McCaskill insisted she was a moderate who could find common ground with Mr. Trump.

“Clearly we can work together on some things,” she said at the debate hosted by KMBC-TV in Kansas City.

She also blamed both parties for the extreme political division in the country. “We’ve got to turn down the temperature,” Mrs. McCaskill said, according to a St. Louis Post-Dispatch report.

Earlier in the debate, however, she echoed a common Democrat attack on the president, saying “I don’t like it that he lies all the time. I don’t get why he feels the need to do that.”

Mr. Hawley said there should be no confusion about his opponent’s partisan allegiance.

“She’s a liberal Democrat,” he said. “It’s a record that doesn’t work for Missouri.”