Despite taking the pledge five months ago that he’d stick with the Republicans regardless of the nominee, Donald Trump nevertheless fired a warning shot Monday at the Republican National Committee – with the threat yet again of an independent run for the White House.  

The Republican presidential front-runner also threatened to sue rival candidate Ted Cruz regarding his Canadian birth if the Texas senator does not retract alleged “lies” about Trump’s policy positions. Further, Trump called on the RNC to intervene in that fight, and said they’d be violating the “pledge” if they don’t.

“The RNC better get its act together,” Trump said at an earlier campaign stop in South Carolina. “I signed a pledge. But the pledge isn’t being honored by them.”

Last September, after much intra-party drama, Trump signed a “loyalty pledge” saying he would support the eventual GOP nominee – though the document was not binding, it was an effort to soothe concerns he might strike out and run as an independent.

Trump is now invoking that pledge on two fronts.

Trump said earlier Monday that the RNC isn’t holding up its end of the bargain, suggesting the committee was hurting his campaign by giving out too many GOP debate tickets to donors. This was a reference to incidents at the last two debates, where Trump repeatedly was booed by the audience – and Trump, in turn, openly accused them of being rival candidate Jeb Bush’s donors and supporters.

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Trump warned Monday that the pledge he signed to support the GOP nominee was "a double-edged pledge."

He then went on to say the RNC would be “in default of their pledge” if they don’t intervene in his dispute with Cruz.

Cruz – on the campaign trail, on the debate stage and in a new campaign ad – has questioned Trump’s positions on abortion, gun rights and more.

In one new ad, the campaign warns conservatives are "just one Supreme Court justice away" from losing on issues that are important to them. Those issues, the ad says, include "life, marriage, religious liberty, the Second Amendment." It suggests Trump would nominate more liberal justices.

Trump said at a press conference Monday afternoon that Cruz is spreading “lies” and he’ll bring a suit challenging the Canada-born lawmaker’s eligibility to run if he doesn’t apologize.

“We will bring a lawsuit if he doesn’t straighten his act out. He’s a lying guy,” Trump said.

"I think he's an unstable person," Trump also said earlier, declaring: "He's nuts."

The criticism comes after Saturday’s bare-knuckle debate where Cruz openly questioned Trump’s pro-life credentials.

“You are the single biggest liar. You’re probably worse than Jeb Bush,” Trump said.

Cruz stood his ground, charging that Trump would “appoint liberals” to the Supreme Court if elected.

Speaking at his own rally in Aiken, S.C., on Monday, Cruz also went after Trump for criticizing George W. Bush’s leadership during and after 9/11, at Saturday’s debate.

Cruz said Trump has sided with MoveOn.org, filmmaker Michael Moore and the “fever swamp left wing.” He called Trump’s slam on Bush – who is hitting the campaign trail Monday for his brother Jeb --  “one of the strangest moments” of the debate.  

The Associated Press contributed to this report.