“The pressure is definitely on, all eyes will be on this,” said Elizabeth Simas, who teaches American politics at the University of Houston and has studied electoral behavior. “The challenge for Rubio is that he’s got to flip the switch and he’s got to get people to focus on electability. The million dollar question is how do you do that?”

During a Wednesday night Republican presidential forum on Fox News, host Megyn Kelly wasted no time in pointing out that he “has not won a single race,” asking what he planned to “change to get a different result” against Trump.

“I’ll admit that he’s the front-runner and that I’m certainly an underdog, but I’ve been an underdog my entire life,” Rubio responded, before tentatively going after the billionaire. “We are not going to let the conservative movement be defined by a nominee who isn’t a conservative. And we certainly need to elect someone who is prepared to be president in a thoughtful and serious and important way given the threats this country faces. So that’s why a majority of Republican voters are not supporting Donald Trump.”