CHAPIN, S.C. — A three-way feud among the GOP’s leading White House contenders escalated Wednesday, with Republican Ted Cruz daring Donald Trump to sue him and dismissing Marco Rubio’s charges of dishonesty just days before South Carolina’s high-stakes primary.

Yet it was Rubio who may have scored the day’s biggest win as he secured the coveted endorsement of South Carolina Gov. Nikki Haley. The popular governor said she was tasked with identifying the best candidate as she surveyed the crowded GOP field.

“If we elect Marco Rubio, every day will be a great day in America,” she said alongside the Florida senator during an evening rally in suburban Columbia.

While a major development, Haley’s endorsement did little to quiet the intensifying clash between Cruz, Trump and Rubio over alleged ethical violations in the days leading up to Saturday’s South Carolina contest.

Cruz, the Texas senator, has been trying to beat back charges of dishonesty from Trump and Rubio for weeks. He shifted his defense to the next level during an afternoon news conference that highlighted Cruz’s extensive legal training.

“You have been threatening frivolous lawsuits for your entire adult life,” said Cruz, a Harvard Law School graduate, speaking directly to Trump. “Even in the annals of frivolous lawsuits, this takes the cake.”

Trump threatened earlier in the week to bring a defamation lawsuit against Cruz over a television ad that attacks the Republican front-runner’s conservative bona fides. Specifically, the ad features footage of the billionaire businessman in a 1999 interview supporting abortion rights. Trump now says he opposes abortion.

Trump’s attorney sent Cruz a letter Tuesday charging the ad was “replete with outright lies, false, defamatory and destructive statements” and saying Cruz could be held liable for damages if it’s not taken down.

Cruz said a lawsuit against the ad has no chance, and said he would like to take Trump’s deposition himself. He also announced plans to run the contested ad more frequently.

“Please Donald, file this lawsuit,” Cruz said during a CNN presidential forum Wednesday night.

The prospective legal battle marks another extraordinary step in the turbulent 2016 Republican primary season.

Polls suggest Trump continues to hold a big lead in South Carolina and in upcoming states, as Cruz works to rally the Republican Party’s most conservative wing and Rubio tries to consolidate mainstream Republicans behind his candidacy. Former Florida Gov. Jeb Bush and Ohio Gov. John Kasich continue to battle for a spot at the table, while retired neurosurgeon Ben Carson struggles for relevancy.