While he disparaged Trump's campaign as "the greatest show on Earth," he saved his most pointed criticism for Cruz.
"This whole notion Ted has that he's the only conservative, I think as people learn more about his record, they'll realize what he really is very calculating," Rubio told CNN's Jake Tapper in an interview that aired Sunday on "State of the Union."
"He's always looking to take whatever position it takes to win votes or raise money, and we're not going to beat Hillary Clinton with someone that will say or do anything to get elected," Rubio said.
He charged the Texan with regularly misrepresenting his immigration positions, particularly on the subject of whether he supports "amnesty," or allowing undocumented immigrants to stay in the U.S.
"I've tried to fix a problem that's a very serious issue in Texas and in Florida, and it's a hard issue," Rubio said. "And clearly we're not going to be able to do it comprehensively and we're not going to be able to do it until we first enforce our immigration laws. But I don't support amnesty. We're not going to have amnesty when I'm president. There's going to be real consequences for violating our law."
Asked by Tapper if Trump could be stopped, Rubio said, "Sure."
"This is a very unusual year. No one is unbeatable. This election is not going to be decided by one or two states. I think the race will narrow after a couple states," he said.
Rubio's comments come one day before Iowans head to the polls in the critical first-in-the-nation caucus state. White House hopefuls of both parties have flocked to the Hawkeye State to make their last-minute pitches to voters, and Rubio is no exception, airing 30-minute television specials
in every media market in the state over the weekend.
But although Rubio placed a distant third in a Des Moines Register/Bloomberg Politics survey
released Saturday night, his campaign feels like it is exactly where it wants to be
headed into Monday night as it bets on emerging as the clear viable alternative to Trump and Texas Sen. Ted Cruz as the campaign continues.
And he doesn't think Trump or Cruz will stand up to voters' scrutiny as the campaign continues.
"Donald's the greatest show on Earth. He's very entertaining, but this campaign isn't about that," Rubio said. "It's got to be about the serious issues confronting the country, because if we get this election wrong, there may be no turning back on some of these issues."