NEWARK, N.J. (AP) — An attorney for Republican presidential candidate Donald Trump has sent what his campaign describes as a "scathing" cease-and-desist letter trying to halt negative ads being run against him.

The letter to the Washington-based Club for Growth claims its new ads portraying Trump as too liberal are "replete with outright lies, false, defamatory and destructive statements and downright fabrications."

"This is the very definition of libel," the attorney, Alan Garten writes.

Trump's attorney takes particular issue with one of the ad's claims that Trump supports higher taxes.

The billionaire businessman once advocated slapping a one-time 14.25 percent tax on individuals and trusts with a net worth of $10 million, but Trump says he has since changed his position.

In his 2000 book, "The America We Deserve," Trump says the one-time revenue-raiser would bring in $5.7 trillion, enough to pay off the entire national debt and allow new tax cuts for middle-class families.

Trump has since retreated from that position and is set to release a formal tax plan, which he says will advocate a major decrease in taxes, in the next week.

The letter calls on the conservative advocacy group, which has said it will spend at least $1 million on TV ads criticizing the Republican presidential front-runner in the early caucus state of Iowa, to immediately stop airing the ads or risk a "multimillion dollar" lawsuit.

Club for Growth president David McIntosh, however, dismissed the letter in a statement and said the group plans to continue airing the ads.

"Tough guy Donald Trump starts whining when his liberal record is revealed," said McIntosh.

"Trump has advocated higher taxes numerous times over many years, just like he's advocated for universal health care, the Wall Street bailout and expanded government powers to take private property," he added. "Trump's own statements prove that our ads are accurate. They will continue to run."

Trump has taken an increasingly hostile tone to his critics in recent days, ramping up his Twitter attacks against Fox News for its coverage of recent polls that show his standing at the front of the GOP field may be starting to crack.

His campaign has also raised eyebrows with its own attacks, including a video posted on the social media site Instagram that suggested former Florida Gov. Jeb Bush, one of his rivals, still smokes pot. Bush admitted during a recent debate to smoking marijuana 40 years ago.

In his own statement, Trump continued to bash the group, which he says is bitter because he refused to give them a $1 million contribution.

"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," he said.

In the letter, Trump's attorney accuses the group of having "openly and shamelessly attempted to extort Mr. Trump to the tune of $1 million in exchange for your political support."