Donald Trump is fighting back against an influential conservative group’s ads that have been running  in Iowa since last week.

A cease-and-desist order sent on Tuesday by Trump’s attorney, Alan Garten, to officials at the Club for Growth details what he says are defamatory “attack ads” that falsely claim Trump supports the “highest tax increase in history.”

In a 30-second spot from the group, which looks to portray Trump as more in step with Democrats than Republicans, he’s slammed for wanting to impose a “14.25% net worth tax on the super rich” to pay the national debt.

The $1-million ad buy by the group began last week and is set to run through next week.

“I am not surprised the dishonest, irrelevant and totally failing Club for Growth has resorted to attacking the definitive front-runner, especially after I refused to contribute to their pathetic group,” Trump, who plans to release a detailed tax policy plan in the coming weeks, said in a statement.

He notes that the ad assails him for a concept he suggested more than a decade ago and has not supported since, which he says makes it libelous.

In a response, David McIntosh, president of the Club for Growth, said the billionaire businessman’s comments make it clear the ads are right. McIntosh’s group is widely viewed as a preeminent institution promoting Republican adherence to free-market and free-trade agendas.

“Trump’s own statements prove that our ads are accurate. They will continue to run. We suggest that Donald grow up, stop whining and try to defend his liberal record,” McIntosh said.

Recent polls since last week’s debate show Trump still polling strong in Iowa, which will hold the first nominating contest in early February.