Lawyers for GOP frontrunner Donald Trump sent Club for Growth a letter Monday demanding that the group cease and desist its ongoing campaign critical of Trump.

The cease and desist demand targets recent TV ads, as well as media interviews promoting the commercials, that suggest Trump is “very liberal.” Monday’s letter singled out a Club for Growth claim that Trump supports higher taxes, which Trump’s legal team says is misleading and comes from a single, 15-year-old article.

“In other words, you lied,” the letter said. “Mr. Trump does not support higher taxes. This is the very definition of libel.”

The letter goes on to accuse Club for Growth of attempting a “shake-down” because Trump refused to donate $1 million to the group in exchange for its support.

“When Mr. Trump, however, presumably unlike many of the other candidates, refused to succumb to your extortionist demands, your only response was to oppose his inclusion in the August 6, 2015 Fox News Republican Presidential Debate, launch a series of misleading Attack Ads targeting Mr. Trump and, ultimately, endorse certain other candidates,” the letter said.

The letter said Club for Growth had the choice of accepting a “one-time opportunity” offer to stop the ads and the related claims, or face a “multi-million dollar lawsuit” against both the organization and its leader, David M. McIntosh.

In a statement Tuesday, Club for Growth dismissed the claims and said it would continue with the campaign targeting Trump.

“Tough guy Donald Trump starts whining when his liberal record is revealed,” McIntosh said in the statement.

Read the letter, released by Trump’s campaign Tuesday, here:

Update: This story has been updated to include a response from Club for Growth.