With just two weeks left until the Iowa caucuses, Donald Trump is escalating his attacks on rival Ted Cruz, whose popularity in the early nominating state currently eclipses the billionaire's support, according to recent polls.

During Saturday's early hours, Trump took to Twitter and fired at Cruz on multiple fronts: from Cruz's "natural-born" eligibility for the presidency to the Texan's debate-night insult of "New York values."

It started with a knock on the U.S. citizenship of the Canadian-born Cruz, which a lawsuit -- filed by a Houston attorney -- has recently called into question:

Ted Cruz was born in Canada and was a Canadian citizen until 15 months ago. Lawsuits have just been filed with more to follow. I told you so — Donald J. Trump (@realDonaldTrump) January 16, 2016

Trump then pivoted to Cruz's neglect in filing papers with the Federal Election Commission about a million-dollar loan he received from Goldman Sachs for his 2012 Senate campaign:

Ted Cruz said he "didn't know" that he was a Canadian Citizen. He also FORGOT to file his Goldman Sachs Million $ loan papers.Not believable — Donald J. Trump (@realDonaldTrump) January 16, 2016

Oh no, just reported that Ted Cruz didn't report another loan, this one from Citi. Wow, no wonder banks do so well in the U.S. Senate. — Donald J. Trump (@realDonaldTrump) January 16, 2016

Trump later hit Cruz for the Texas senator's slam against "New York values" during Thursday's GOP debate in South Carolina.

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On Friday, after several Democrats banded together behind Trump over his debate stage defense of New York City, Cruz gave an "apology" for his remarks.

"So, today, Donald Trump and Hillary Clinton and Governor Andrew Cuomo and Mayor Bill de Blasio of New York City all demanded I apologize....Who am I say to no? I will apologize," the Texas senator said in South Carolina. "I apologize to the millions of New Yorkers who have been abandoned for years by liberal politicians...I apologize to all the New Yorkers who are pro-life, pro-marriage and pro-Second Amendment, who were told by their governor -- Governor Cuomo -- that there is no place for them in state of New York because that's not what New Yorkers believe."

But Trump called out his rival's latest comments on the issue, blasting them as a "wiseguy apology":

The Ted Cruz wiseguy apology to the people of New York is a disgrace. Remember, his wife's employer, and his lender, is located there! — Donald J. Trump (@realDonaldTrump) January 16, 2016

And then Trump took to speculating about Cruz's other possible business dealings:

Based on the fact that Ted Cruz was born in Canada and is therefore a "natural born Canadian," did he borrow unreported loans from C banks? — Donald J. Trump (@realDonaldTrump) January 16, 2016

Ted Cruz purposely, and illegally, did not list on his personal disclosure form personally guaranteed loans from banks. They own him! — Donald J. Trump (@realDonaldTrump) January 16, 2016

The once-cordial relationship between the two candidates has rapidly soured in recent weeks, as Cruz has overtaken the GOP front-runner in Iowa and is closing in on Trump's lead in New Hampshire.

On Saturday, responding to Trump's social media assault, Cruz derided Trump's newfound "nervous energy."

"It seems Donald doesn't react well when he's going down in the polls," Cruz told reporters in South Carolina. "Knowing Donald that has got to drive him nuts - got him out of bed this morning and tweeting and tweeting and tweeting."

"You know, in terms of a commander-in-chief, I think we ought to have someone who isn't springing out of bed and tweeting in response to the latest polls," the Texas senator continued. "Stable and steady and a calm hand to keep this country safe."