Donald Trump declared on Monday that he will file a lawsuit over Ted Cruz's eligibility for the White House if his Republican presidential rival does not remove television ads against him.

Taking the largely discredited line that Cruz's Canadian birth disqualifies him from the American presidency, Trump said that "Cruz has become unhinged and is lying" about Trump's record on abortion and gun rights, among other issues.

"One of the ways I can fight back is to bring a lawsuit against him relative to the fact that he was born in Canada and therefore cannot be president," Trump said. "If he doesn’t take down his false ads and retract his lies, I will do so immediately."

Cruz responded on Twitter on Monday afternoon:

You cannot simply scream "liar" when someone points out your actual positions: https://t.co/DI5HnSln62 — Ted Cruz (@tedcruz) February 15, 2016

Fellow Republican presidential candidate Marco Rubio launched a similar attack on Cruz Monday, blasting out an email titled, "Cruz BusTED On Lies."

"Senator Cruz will say or do anything to win an election including employing underhanded tactics and making charges against all his opponents that he knows are outright lies. America can’t afford more politicians like Ted Cruz who will easily sacrifice principle for political gain," said Joe Pounder, a Rubio spokesperson, in the email.

This comes after South Carolina TV stations pulled ads from a pro-Cruz Super PAC after a legal review. The ad attacked Rubio's immigration record and alleged the Florida senator's support for "sanctuary cities."

But though Trump and Rubio are joining forces in ganging up on Cruz, questioning the Americanness of others has been a staple of Trump's presidential campaign — be it over Mexican immigrants, Muslims, or his fellow presidential contenders.

Trump's threat to sue Cruz over his presidential eligibility is a purely retaliatory gesture over the ads, which appear to be an unrelated issue. There are broad First Amendment protections for speech about public figures, and it's unlikely Trump could win a case against Cruz in court.

It should also be noted that this is unlikely to prove an effective deterrent for Cruz's campaign, given that Trump's supporters have already filed a lawsuit over Cruz's eligibility, according to the Hill.

In his statement, Trump also called on the Republican National Committee to do something to stop Cruz's attack ads.

"Additionally, the RNC should intervene, and if they don’t they are in default of their pledge to me," Trump said.

Trump signed a pledge not to launch an independent third-party bid for the White House if he lost the Republican nomination, but that was contingent on the RNC acting fairly to him. How to act fairly by Trump's standards, however, was left undefined.