'Mr. Meltdown': Donald Trump attacks Marco Rubio after the debate

Donald Trump has been training most of his fire on his rival Ted Cruz over the past few weeks, but after Thursday night's debate, he has a new target: Marco Rubio.

The real-estate mogul and Republican presidential frontrunner went after Rubio on Twitter on Friday morning, calling him "Mr. Meltdown."

"Lightweight Marco Rubio was working hard last night," Trump tweeted. "The problem is, he is a choker, and once a choker, always a chocker! Mr. Meltdown."

Rubio and his fellow Republican presidential candidates went after Trump aggressively and often at the CNN debate in what turned out to be the biggest Trump pile on yet. Rubio and fellow Sen. Cruz each went on the attack against Trump over several issues including taxes, immigration, Trump's history as a reality-television host, and his business career.

Responding to a Trump attack about his relationship with lobbyists, Rubio said: "Here's a guy who inherited $200 million. If he hadn't inherited $200 million, you know where Donald Trump would be? He would be selling watches in Manhattan."

The repeated clashing came as both Cruz and Rubio are staring far up at Trump in the race for the Republican nomination. Trump is the clear frontrunner in the primary contests, having won the New Hampshire and South Carolina primaries and Tuesday's Nevada caucuses. Rubio, along with Cruz, has been fighting to take Trump down and establish himself as the clear alternative.

But until now, Trump has focused mostly on attacking Cruz, whom he has branded a "liar." The attacks seem to have had an effect on Cruz's campaign. Cruz recently fired his campaign's communications director after he spread a video that purportedly showed Rubio dismissing the Bible, emphasizing that he is conducting his campaign "with the very highest standards of integrity."

So far, Cruz is the only one who has beaten Trump in a primary or caucus contest. He narrowly won the Iowa caucuses, finished third in South Carolina, where he was expected to do well.

Early Friday morning, Trump also retweeted some supporters criticizing Rubio: