Ted CruzRafael (Ted) Edward CruzSenate Republicans offer constitutional amendment to block Supreme Court packing 10 bellwether counties that could signal where the election is headed Conservatives seize on New York Post story to push Section 230 reform MORE on Wednesday assailed Donald TrumpDonald John TrumpNearly 300 former national security officials sign Biden endorsement letter DC correspondent on the death of Michael Reinoehl: 'The folks I know in law enforcement are extremely angry about it' Late night hosts targeted Trump over Biden 97 percent of the time in September: study MORE for threatening legal action against his campaign, unloading on the Republican front-runner ahead of the South Carolina primary.

The Texas senator revealed that the Trump campaign has sent him a cease-and-desist letter over an ad running in South Carolina that paints the businessman as supportive of abortion rights. That ad features a clip from 1999 where Trump calls himself  “very pro-choice.”  

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After reading from the cease-and-desist letter with pointed emphasis, Cruz dared Trump to follow through.  

"I have to say to Mr. Trump — you have been threatening frivolous lawsuits for your entire adult life, even in the annals of frivolous lawsuits, this takes the cake," Cruz told reporters at a press conference in South Carolina. 

"Donald, I would encourage you, if you want to file a lawsuit challenging this ad, claiming it is defamation, file the lawsuit ... The operative words in that ad come from Donald Trump's own mouth.” 

Trump quickly fired back at Cruz, calling the attacks “desperate moves by a guy who is tanking in the polls.”

“He is a liar and these ads and statements made by Cruz are clearly watching his campaign go up in flames finally explains Cruz’s logo," he said in a statement.  

“If I want to bring a lawsuit it would be legitimate. Likewise, if I want to bring the lawsuit regarding Senator Cruz being a natural born Canadian I will do so. Time will tell, Teddy.”  

The back-and-forth comes after a contentious stretch of the campaign where Cruz has faced attacks from all sides. Trump has repeatedly accused Cruz of stealing the Iowa caucuses and called him "unstable" and "nasty," while Marco RubioMarco Antonio RubioOwners of meatpacker JBS to pay 0M fine over foreign bribery charges Questions raised about conflicts of interest around Biden son-in-law America needs an industrial policy — now more than ever MORE cast Cruz as a liar during a heated argument during Saturday's debate.  

The incoming fire appears to have taken a toll in South Carolina, a state that had been seen as favorable territory for Cruz due to its conservative and evangelical bent. 

But new polls released this week show Cruz trailing by double digits, with Trump poised for a huge win in the state on Saturday. Wednesday's Monmouth University poll gives Trump 35 percent support, nearly doubling Cruz, who garnered just 19 percent.

Trump's lead extends to the very evangelical community that Cruz had expected to win in large numbers. 

Looking to fight back, the Cruz campaign made an appeal to the religious right in a new a video where the prominent Christian conservative Dr. James Dobson explains his support for Cruz.

“For us, the sanctity of human life and traditional marriage is non-negotiable. Ted has been relentless in defending the institution of the family,” Dobson said.

During his press conference, Cruz laid out a heated point-by-point argument about Trump’s past positions on abortion, a key issue for religious conservatives. He noted that Trump said last year that his sister would make a "phenomenal" Supreme Court justice.  

"Who is his sister?" Cruz asked. 

"A court of appeals judge appointed by Bill ClintonWilliam (Bill) Jefferson ClintonLate night hosts targeted Trump over Biden 97 percent of the time in September: study A closing argument: Why voters cannot trust Trump on healthcare On India, the US must think bigger MORE. His sister has an extreme left-wing record on the issue of abortion."

The Supreme Court has become a focal point of the presidential race following the death Saturday of Justice Antonin Scalia. 

The Cruz ad warns voters they can’t trust Trump with appointing justices in light of his past pro-choice comments. 

Cruz ran down a list of liberal politicians who have received donations from Trump, including Vice President Biden, Chicago Mayor Rahm Emanuel and former secretary of State Hillary ClintonHillary Diane Rodham ClintonLate night hosts targeted Trump over Biden 97 percent of the time in September: study 10 steps toward better presidential debating Continuity is (mostly) on the menu for government contracting in the next administration MORE

While he noted that Trump defends those donations by arguing he donated to politicians to get his way, Cruz said his giving helped Democrats win back the House in 2006. 

"Donald Trump is a direct cause of ObamaCare passing because if either house had remained in Republican control, ObamaCare would not have passed," Cruz said. 

"Donald helped fund the Democratic takeover of both houses."

In goading Trump to file his lawsuit, Cruz warned Trump that he himself would have to be deposed under oath.

"I may well not use outside counsel, may take the deposition myself," Cruz said. 

"Whether in a deposition or court of law, getting Donald Trump under oath, under penalty of perjury, answering these questions, I'd point out it didn’t work out very well for Bill Clinton." 

Cruz also used the moment to slap at Rubio, accusing him of obfuscating his personal record during Saturday's debate..

"Marco Rubio is behaving like Donald Trump with a smile," he said. 

- This story was updated at 1:48 p.m.