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Donald Trump launches all-out assault on Ted Cruz

Republican U.S. presidential candidate businessman Donald Trump (L) and Senator Ted Cruz speak simultaneously at the Fox Business Network Republican presidential candidates debate in North Charleston, South Carolina, January 14, 2016.

Randall Hill/Reuters

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:

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:

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

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":

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

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."