Marco Rubio: If Donald Trump is nominee, voters will "tune out"

Fresh off a strong -- and civil -- debate performance in Florida Thursday night, Marco Rubio is predicting a stinging defeat for the Republican party in November if the current GOP front-runner, Donald Trump, becomes the nominee.

"There is a very significant number of Republicans that will never vote for him and you can't win unless the party's united," Rubio told "CBS This Morning" in an interview that aired early Friday. "If he's our nominee, I think we will lose."

"There's a significant number of people that will just not vote," he said, anticipating a "fracture" in the party during the general election. "They'll just tune out."

Still, when asked by "CBS This Morning" co-host Charlie Rose if the party could "never" unite behind Trump, Rubio responded: "You never say never."

The Florida senator also expressed skepticism of Ted Cruz, Rubio's other rival and the current runner-up to Trump in the race for delegates.

Trump applauds "very civil," "elegant" Florida debate

"I think Ted too will struggle to unite the party, to be honest," he said. He noted that a nominee would have to be a general election candidate that Republicans can "coalesce around quickly" but also "someone who gives us a chance to grow the party."

"I don't think Ted can grow the party," Rubio said, warning that Cruz will spend more time as the nominee "simply trying to keep the Republican party together."

In Florida specifically, where nearly 100 delegates will be at stake during Tuesday's winner-take-all primary, Rubio forecasted low turnout for backers of Cruz and Ohio Gov. John Kasich.

"There's a growing realization in Florida among both supporters of John Kasich and Ted Cruz that they can't win," he said. "A vote for them is a vote for Donald Trump... you're going to see a lot of people coming our way here down the stretch."

Rubio, who so far trails behind Trump by over 300 delegates, believes his home state primary will turn the tide in his favor.

"I really like the way the map looks moving forward in terms of the delegate count," he said. "But it all begins next Tuesday, and I feel really good about the progress we're making."

He gave a similarly positive view of Thursday's debate in Miami, where a majority of the questions focused on policy issues.

"That's the kind of debate we wanted to have...so we felt really good about it," Rubio said. "Voters hopefully get to see what I would do on these issues as president. I don't think Donald did that but I'll let voters make that assessment."