Republican US presidential candidate Donald Trump gives the thumbs up as he departs his campaign rally in Portland, Maine March 3, 2016. [Photo/Agencies]

Indeed, Rubio has in recent days adopted a more aggressive stance toward Trump, billing him as a "con artist" in several interviews and speeches, although many analysts say the tactic is too little and too late.

"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 Mar. 15 primaries.

"(Rubio) may be heartened by results in Minnesota, where he won, and in Virginia, where he closed the polling gap to come in a close second place," Mahaffee added, referring to Super Tuesday.

Trump's other main rival, Ted Cruz, is also holding on.