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Trump slams Rubio as 'very weak' on immigration

But the billionaire's insults may be evidence that Rubio is emerging as a threat.

Sen. Marco Rubio is the latest Republican presidential candidate to find himself in the crosshairs of frontrunner Donald Trump. But he may fare better than some other candidates have.

On Friday afternoon, Trump took to Twitter to slam Rubio on immigration, as well as on his attendance record in the Senate.

Then Saturday morning, Trump went back for more:

Trump’s tweets are the latest insults in a feud with Rubio that has played out for much of the past week. But unlike Trump's criticism of other candidates, this one may not work out to the billionaire’s advantage. In fact, the skirmish appears to reflect Rubio’s growing prominence in the race, after the departure of Wisconsin Gov. Scot Walker, and Jeb Bush’s ongoing struggles on the campaign trail.

At the Values Voters Summit on Friday, Trump called Rubio a “clown” — a remark that drew boos from the conservative crowd. Trump also accused Rubio of “changing his tone” on immigration after he went down in the polls.  And later, Trump told reporters he thinks the senator is “a baby.”

RELATED: Conservatives boo Donald Trump for criticizing Marco Rubio

In 2013, Rubio led an effort in the Senate to provide a path to citizenship for undocumented immigrants, but abandoned it amid intense opposition from grassroots conservatives. Still, he's not nearly as reviled among Republican base voters as Bush, who was the main target of Trump's attacks until recently.

On Wednesday, Trump went after Rubio for running up credit card debt, saying: “He’s got no money — zero.”

Unlike some of the previous targets of Trump’s attacks, Rubio has responded in the same highly personal vein, and has seemed to hold his own.

“He had a really bad debate performance last week,” Rubio said on a Kentucky radio show Thursday, referring to Trump. “He’s not well informed on the issues. He really never talks about issues and can’t have more than a 10-second sound bite on any key issue. I think he’s kind of been exposed a little bit over the last seven days and he’s a touchy and insecure guy. So that’s how he reacts and people can see through it.”

And on Friday, Rubio took that message to Fox News, saying Trump “had a really bad debate. Carly Fiorina embarrassed him.” Rubio also mocked Trump for appearing in front of “an empty room” at a recent campaign event, a reference to reports that a Trump campaign event was lightly attended.