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Inside Donald Trump's feud with Marco Rubio

For months, Donald Trump and Marco Rubio pretty much ignored each other. Now, they are in the thick of an ugly spat. Here's why the peace broke down.

After largely ignoring Marco Rubio for months, Donald Trump suddenly can’t heap enough criticism on the Florida senator. Trump tweeted that Rubio's middle name was “amnesty” on Tuesday; he called Rubio a “lightweight” in South Carolina on Wednesday; and he offered up some physical observations about the senator on msnbc’s “Morning Joe” Thursday. 

“[He] has the worst voting record in the United States Senate,” Trump said. “He sweats more than any young person I've ever seen in my life.”

Then, Trump criticized Rubio again Thursday night, slamming the senator's Iowa event on Twitter.

Now, there’s nothing new about Trump dumping on another candidate. It’s kind of all he does. What was surprising was that Rubio, who has normally avoided these kinds of spats, responded with vigor.

RELATED: Rubio slaps back at 'touchy' Trump

"He had a really bad debate performance last week," Rubio said in a appearance on Kentucky Sports Radio Thursday. "He's not well-informed on the issues. He really never talks about issues and can't have more than a 10-second soundbite on any key issue. And I think he's kind of been exposed a little bit over the last seven days, and he's a very touchy and insecure guy and so that's how he reacts, and people can see through it." 

This marks the first time that Rubio and Trump have gotten into an extended back-and-forth and, in many ways, it makes sense for both of them. Here are some theories as to why their long détente might finally be breaking down.

Why would Trump go after Rubio?

1. It gets him out of other fights.

Trump’s default strategy is to go on offense; the only thing that changes is the target. If one attack doesn't go as planned, he shifts his attention elsewhere quickly. One typical example: In July, Trump drew his first real push-back from Republicans after he argued that Sen. John McCain was “not a war hero” because he was captured in Vietnam. Rather than apologize or dwell on the episode, Trump gave out Sen. Lindsey Graham’s phone number in a speech. Boom, McCain fight is old news.

Last week, Trump’s principle antagonist was Carly Fiorina, with whom he clashed repeatedly during the GOP primary debate making deragoatory comments about her appearance to Rolling Stone magazine. Unlike prior fights with his rivals, Trump – who has well-documented issues addressing women -- seemed uneasy going up against Fiorina, who earned a bump in the polls from the debate while Trump’s lead shrank. 

RELATED: All the people Trump has insulted in September so far 

Fiorina, who on Thursday approvingly quoted National Review editor Rich Lowry’s suggestion that she had castrated Trump at the debate, clearly wants to keep up the feud. Trump, on the other hand, is suddenly scrapping with Rubio along with Fox News and Club For Growth. Chances are, it's not a coincidence.

2. Rubio may be the best target.

U.S. Republican presidential candidate Donald Trump gives an address at the South Carolina African American Chamber of Commerce in North Charleston, SC., Sept. 23, 2015. (Photo by Randall Hill/Reuters)
U.S. Republican presidential candidate Donald Trump gives an address at the South Carolina African American Chamber of Commerce in North Charleston, SC., Sept. 23, 2015. 

For one, Fiorina and Carson have their own troubles to worry about. They’re outsider candidates who are only now getting the kind of scrutiny that some of the more experienced contenders (like Rubio) are used to. Fiorina’s checkered business record is under the microscope along with remarks she made in 2013 supporting a requirement that Americans buy health insurance, a core feature of Obamacare. Carson is now dealing with stories about his attitudes towards Muslim-Americans that raise questions about whether he’s mainstream enough for a general election. 

WATCH: Trump on Fox, Clinton, Rubio and birther issue

Plus, while Fiorina and Carson may out-poll Rubio for now, many political insiders see Rubio as the most likely nominee. He’s broadly well-liked in polls of GOP voters, his biography matches up well against Hillary Clinton's, and he’s well-positioned to capitalize on Scott Walker’s exit – already a number of key fundraisers are reportedly moving to his campaign.

Trump’s best bet may be to knock him out early before he can gain momentum, especially given that Rubio’s number one vulnerability is his role crafting the 2013 Senate immigration bill that included a path to citizenship for undocumented immigrants. Trump's signature issue is his hardline stance on immigration, so he seems uniquely positioned to attack Rubio. For now, though, he seems more interested in talking about his precious bodily fluids instead.

2. Why would Rubio go after Trump? 

He senses weakness.

For months, Rubio has largely stayed above the fray when it comes to Trump. In fact, he has largely stayed above the fray, period. While Rubio’s rivals have chased shiny objects each news cycle, he has mostly campaigned in the background and hung around the middle of the polling pack while waiting for the right moment to pounce. 

RELATED: Trump stirs feud ahead of campaign swing

That right moment might be arriving, though, especially where Trump is concerned. In several polls since Wednesday’s debate, the trend is an abrupt drop for Trump – the first real sign of vulnerability since he first took over the race. A RealClearPolitics average puts Trump in first place with 24% support, a significant drop-off from his pre-debate peak of 30.5%.

Trump’s response so far has been a whole lot of whining about which polls are the correct ones, which is not a great look for someone whose whole campaign is built on winning. Perhaps Rubio thinks Trump’s off balance enough that the right push could send him toppling over.

He’s learned from Jeb Bush’s mistakes.

Trump spent weeks spewing bile at Jeb Bush, whom he mocked as a “low energy” retread of his presidential brother and father. Bush did his best not to get dragged down into the mud, but eventually his campaign concluded that the attacks were doing real damage when left unchallenged and made their candidate look weak. Bush pivoted towards a policy of aggressive confrontation on the campaign trail and came ready for war in the second debate.

Now Trump is trying to paint Rubio as a nervous "lightweight" who wants to impose “amnesty” on the country. This time, however, Rubio is responding with overwhelming force, including the kind of personal insults that are usually Trump's domain. If the goal is to avoid replicating Bush’s slow response to similar character assaults, this sure seems like the way to go about it.