Responding to threats of a lawsuit from Donald Trump over a campaign ad, Texas Sen. Ted Cruz went into full lawyer mode, delivering a four-point defense to reporters in South Carolina Wednesday afternoon.

Standing up in front of the room with papers on the table in front of him and charts behind him, Cruz put his college debating skills to use and explained why a Trump lawsuit would be "frivolous."

"Even in the annals of frivolous lawsuits, this takes the cake," Cruz said. "So Donald, I would encourage you if you want to file a lawsuit challenging this ad, claiming it is defamation, file the lawsuit. It is a remarkable contention that an ad that plays video of Donald Trump speaking on national television is somehow defamation."

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Trump's legal threats came in response to an ad Cruz's campaign released in the wake of the death of Supreme Court Justice Antonin Scalia, which features old video footage of Trump saying he is "very pro-choice" and would not ban partial-birth abortions. "We cannot trust Donald Trump with these serious decisions," the ad says.

In response, Trump called Cruz "unstable" and said he was "seriously" considering legal action against Cruz for the ad.

At Wednesday's press conference, Cruz said Trump's own past words and actions demonstrate that the ad's claims are true. Laying out his case, Cruz cited four pieces of "evidence": the 1999 interview with Tim Russert featured in Cruz's ad; Trump's assertion last summer that his sister, Maryanne Trump Barry, would make a "phenomenal" Supreme Court judge; Trump's past campaign contributions to Democrats; and his statement during a previous GOP debate that Planned Parenthood "does do wonderful things."

Cruz blasted Trump for his past campaign contributions to candidates like former President Jimmy Carter, former secretary of State Hillary Clinton and Vice President Joe Biden, whose names were listed on a chart among the other Democrats Trump has donated to.

"We know as a fact that Donald Trump does not care about conservative Supreme Court justices...because no one who cares about conservative Supreme Court justices would make contributions to these candidates."

Cruz also took the media opportunity to go after Marco Rubio, another rival who's been attacking him on the stump in the final days before the South Carolina primary.

"His campaign has not gotten the traction he wanted," he said of the Florida senator. "...And the consultants who are running his campaign have apparently made the decision that their one hope of doing well is to make false attacks with no basis whatsoever."

Tying Trump and Rubio together, Cruz said both candidates' accusations against him are "not indicative of candidates running honest campaigns."