President Trump’s campaign is threatening to sue over an alleged pre-payment demand ahead of his Thursday rally at Target Center in downtown Minneapolis.

The president’s campaign sent a letter to Target Center management Monday after they were allegedly told they needed to pay $530,000 ahead of the rally for security costs and other fees. Without payment, the Target Center would “withhold use of the arena,” according to the letter.

The Target Center is owned by the city of Minneapolis but run by a private company, AEG Worldwide. Trump’s campaign is threatening legal action as early as Tuesday morning if officials block the rally without payment of security fees.

The campaign blamed the sudden move on political opposition to the president by Minneapolis Mayor Jacob Frey, a Democrat.

“We presume this contrived legal justification for threatening to terminate the contract is the result of AEG receiving pressure from city leadership determined to cancel the rally,” read the letter from Trump law firm Jones Day to AEG. “We are well aware of Mayor Frey’s vocal partisan opposition to President Trump and calls to disrupt the rally.”

Frey’s office did not respond to a request for comment Monday night, but he responded to a tweet from Trump and his campaign Tuesday morning.

In a statement after Trump’s rally was announced, Frey said he couldn’t legally block the president from coming but his “message of hatred will never be welcome in Minneapolis.”

Trump’s rally is expected to draw huge crowds on Thursday, both inside the arena as well as protesters outside. City officials have been mum publicly about the anticipated security costs.

AEG did not immediately respond to request for comment.

The campaign often does not reimburse local governments for costs associated with his rallies.

According to the campaign, the Secret Service is “solely” responsible for securing the arena and blocking the rally would be a breach of their contract.