ST. LOUIS — The incredibly tight Senate race in Missouri is flooded with more outside money than any other midterm contest in the country, and it is financing a ruthless TV ad war that has soiled the images of both Democrat Sen. Claire McCaskill and Republican challenger Josh Hawley.

More than $41 million from outside groups has churned out a barrage of TV and radio ads, most of them negative, according to campaign finance data compiled by the Center for Responsive Politics.

The next highest level of outside spending is the $31 million thrown at Florida’s Senate race.

In Missouri, Mrs. McCaskill and Mr. Hawley, the state attorney general, are locked in a virtual tie in a race that could determine which party controls the Senate.

Slightly more of the outside money came from groups backing Mr. Hawley. A little more than $21 million supports the challenger, and nearly $20 million is for Mrs. McCaskill, according to the center’s data.

According to the TV ads, it is a contest between two despicable characters: Mrs. McCaskill, a suspiciously wealthy Washington insider who would rather funnel federal money to her husband’s development company than help domestic abuse victims; and Hawley, a political climber in the pocket of corporate special interests who is ready to do their bidding at the expense of Missourians.

The effect, however, appears at best to be a draw. The candidates have been running neck and neck in polls since early in the race.

“I think most of the time they make up stuff. It’s not convincing me,” said Kermit Beeks, 65, a part-time worker at the St. Louis Public Library. “I just turn the channel.”

Dave Robertson, chairman of the political science department at University of Missouri-St. Louis, said neither candidate is winning the ad wars.

“Lots of TV ads are pummeling Missouri voters,” he said. “I don’t think they have changed the race that much.”

However, he said both sides have scored some points.

The groups behind the spending underscore what is at stake, with Democrats needing a net pickup of two Senate seats to win the majority and Mrs. McCaskill fighting to hold on in a state President Trump won by 19 points in 2016.

Senate Majority PAC, a group run by allies of Senate Democrat Leader Charles E. Schumer, spent the most at more than $11.7 million.

Nearly $3 million more came from the Senate Majority PAC’s nonprofit offshoot Majority Forward.

On the other side, Senate Leadership Fund, a super PAC controlled by allies of Senate Majority Leader Mitch McConnell, a Kentucky Republican, put up $8 million.

At least 17 Republican or conservative groups and six Democrat or liberal groups ponied up cash for the contest, according to the Center for Responsive Politics.

The most effective ads for Mrs. McCaskill highlight Mr. Hawley’s participation in a lawsuit challenging the Affordable Care Act that threatens coverage of pre-existing conditions.

“Why would Josh Hawley punish those whose only crime was getting sick? Because he’s for the insurance companies, not us,” said the voiceover in an ad produced by Senate Majority PAC.

The most effective ads for Mr. Hawley hammer home Mrs. McCaskill’s status as a longtime incumbent.

An ad by Senate Leadership PAC slams Mrs. McCaskill for backing amnesty for illegal immigrants and supporting sanctuary cities that are “havens for illegal immigrant criminals.”

The announcer in the spot says Mrs. McCaskill has “gone Washington and left Missouri behind.”

Mr. Robertson said that was a powerful message.

“Incumbency, once again, is a bit of a burden for congressional candidates, all things being equal,” the professor said.

The question remains whether Mrs. McCaskill and Mr. Hawley are on equal footing.

The president’s popularity has slid since 2016. A Fox News poll last week found Mr. Trump’s job approval was still above water in Missouri at 54 percent. But that’s several points off the 56.8 percent of the vote he captured in 2016.

Mr. Robertson also noted that despite Mr. Trump’s 19-point margin in 2016, the state’s incumbent Republican Sen. Roy Blunt won by less than 3 percent on the same ballot.

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