It takes 1,237 delegates to win the Republican nomination for president.
The Republican's most recent presidential nominees, Mitt Romney and John McCain, lambasted current front-runner Donald Trump on Thursday, calling him unfit for office and a danger for the nation, in an extraordinary show of intra-party chaos.
Rubio justified his attacks on Trump by saying the billionaire businessman had "basically mocked everybody" over the past year.
That was the last question at Thursday's GOP debate in Detroit. Trump said during the debate, raising his hands in the air so TV and live viewers could see.
"Rubio will stay in the race, at least through Florida, hoping that anti-Trump voters coalesce behind him", Dan Mahaffee, an analyst with the Center for the Study of the Presidency and Congress, told Xinhua, referring to the Florida senator who hopes to win his home state on the March 15 primaries.
It's also the first time he'll face questioning from Fox News' Megyn Kelly since the two clashed in the first primary debate.
And more broadly, Trump insisted that compromise would be part of any immigration reform.
"I'm not running for president and I won't run for president", Romney said. "Donald Trump whipped the establishment and it is too late for the limp GOP establishment to ask their mommy to step in and rewrite the rules because they were humiliated for their impotence".
Romney advisers have recently sought information on a contested convention, though there appeared to be no concrete planning for that possibility and it was unclear whether their efforts signaled Romney's own interest in becoming the GOP nominee through a floor fight, according to a Republican familiar with the efforts.
Randy Barnett, a professor at Georgetown University's law school, has proposed that Cruz and Rubio form a partnership, with each pledging to support the other at the convention.
Billionaire Donald Trump writes his own checks to pay for his campaign. But in Thursday's chaotic debate, he signalled his willingness to compromise on immigration, among other key issues.
At center stage, Trump, 69, defended himself from criticism earlier in the day from 2012 Republican nominee Mitt Romney and faced further questions about his business record.
And his path forward relies not so much on winning states outright as gaining on Trump in the delegate count in states that allocate them proportionally to prevent him from getting the majority needed to win the nomination outright. Mr. Romney said on Thursday "Trump says he admires Vladimir Putin while he has called George W. Bush a liar". To the suggestion that he might be small elsewhere, Trump said: "I guarantee you there is no problem".
Cruz and Rubio, in particular, aggressively sought to deflate Trump, fiercely attacking him as a false conservative, an ill-tempered bully and a flip-flopper on issues.
"The overall response that we've gotten has been pretty negative towards Romney", said Matthew Bramin of the Kern County Young Republicans. He joined a growing list of Republican leaders aggressively working to stop the former reality television star, who they claim is not a true Republican and lacks the experience and knowledge to lead the world's most powerful nation.