SPRINGFIELD, United States (AP) — Republican US Senator Mark Kirk said President Barack Obama was “acting like the drug dealer in chief” when his Administration delivered US$400 million in cash to Iran contingent on the release of American prisoners — the latest in a string of provocative comments by the Illinois senator caught in a tight re-election race.

Kirk, whose race against Democratic US Rep Tammy Duckworth could help determine control of the Senate next year, made the remarks during an editorial board meeting last week with The (Springfield) State Journal-Register.

“We can’t have the president of the United States acting like the drug dealer in chief,” Kirk told the paper, “giving clean packs of money to a … state sponsor of terror.”

Republicans have criticised the Obama Administration after it acknowledged that the repayment of the money from a 1970s Iranian account was connected to a US-Iranian prisoner exchange in January. The Administration denies the money was “ransom”, saying the Islamic Republic would have soon recouped the money one way or another.

A spokesman for Duckworth, Matt McGrath, said Kirk should apologise. “Senator Kirk’s comments are misguided and deeply offensive, and beneath the dignity of the office he holds,” McGrath said in a statement.

Kirk’s campaign defended his comments, calling the actions from the Obama Administration “reckless in the extreme”. In a statement, Kirk spokesman Kevin Artl characterised the transaction with Iran as “more representative of nefarious deals than the conduct of the world’s greatest democracy”.

Kirk, who is chairman of the Senate Subcommittee on National Security and International Trade and Finance, is planning a hearing to discuss the payment next month, his campaign said.

The Democratic National Committee, meanwhile, called Kirk’s remarks “racially tinged” and likened them to Republican presidential nominee Donald Trump’s “hateful rhetoric”. Kirk is not endorsing Trump and has called him “bigoted and racist”.

“In calling our first African-American president a drug dealer, Mark Kirk has demonstrated that Donald Trump’s hateful rhetoric and divisive politics have taken over the entire Republican Party,” DNC spokesman TJ Helmstetter said in a statement.