A top campaign aide for billionaire Tom Steyer privately offered local Iowa politicians campaign contributions in exchange for their endorsements in the 2020 Democratic presidential race, the Associated Press reported, citing people with direct knowledge of conversations between the aide, Pat Murphy, and the local politicians.

Payments for endorsements would violate campaign finance laws if they weren't disclosed to the Federal Election Commission, according to AP. There is no evidence that any local politicians accepted the money from Murphy or Steyer's campaign.

When asked about the AP report, Steyer's campaign responded with a statement from Murphy, a former Iowa House speaker, who did not directly address the allegations.

"As a former legislator, I know how tricky the endorsement process can be for folks in Iowa. It was never my intention to make my former colleagues uncomfortable, and I apologize for any miscommunication on my part," Murphy said in a statement to CNBC.

"I joined the campaign because I believe Tom is the best candidate to take on Donald Trump and that he shares Iowa's values. I know that Tom's message will resonate with leaders across the state and that any endorsements will come from the merit of his message."

Since launching his campaign on July 9, Steyer has spent almost $30 million on television ads, including $7.1 million in Iowa alone.

Some of Steyer's Democratic rivals have complained that he has essentially bought his way into the election.

Montana Gov. Steve Bullock criticized Steyer on Thursday after the report came out, saying in a tweet, "You buy your way to the debate stage. You try to buy your endorsements. @TomSteyer, we won't be bought."

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In late October, former Secretary of Housing and Urban Development Julian Castro repeatedly criticized Steyer in emails to his own supporters, accusing the billionaire of pumping $47 million into his campaign coffers. Castro's campaign did not immediately return a request for comment from CNBC.

Presidential candidates and Sens. Bernie Sanders and Elizabeth Warren have argued for wealth taxes targeting millionaires and billionaires to fund their policy goals. Neither Sanders' nor Warren's campaigns immediately responded to requests for comment on the news about Steyer.

"Tom has not made any individual contributions to candidates in Iowa this year, and he will not be making any contributions," a campaign spokesperson for Steyer said in a statement to CNBC.

"The endorsements he receives are earned because of Tom's campaign message, his decade-long work taking on big corporations who put profits over people, and his work registering and organizing voters across the country to support progressive causes. Our campaign policy is clear that we will not engage in this kind of activity, and anyone who does is not speaking for the campaign or does not know our policy."

Steyer is polling at 3% in Iowa, according to the latest Quinnipiac University poll.