Republican presidential candidate Marco RubioMarco RubioObama: Trump's rigged election talk 'not a joking matter' Obama: Trump and Putin have a 'bromance' Obama slams Rubio for Trump support MORE on Sunday would not say whether it is time to directly attack front-runner Donald TrumpDonald TrumpTrump: 'Very disappointed' GOP senator dropped support Trump steps up campaign spending in final stretch Clinton and Trump chat at Al Smith dinner MORE.
Up to this point, Rubio said, there have been several candidates dividing 70 percent of the vote.
"And so we had a very unusual circumstance. I was being attacked from all sides. I mean, we had one super PAC that spent $40 million going after me. So, you can only take on so many people at one time," he said.
Rubio said it's important to remember though that about 70 percent of the Republican party nationally is not voting for the front-runner. Once the number of candidates narrows, he added, it'll be a "different election."
"And we're getting closer to that point."
He emphasized that the nominee needs to bring the party together, so it can grow, spread its message and win in November.
"We cannot lose this election. And I give our party the chance to nominate someone as conservative as anyone in this race. But I am a conservative that can unite us, that can grow this party, and that can ultimately win in November," he said.
"And that's the argument we're going to be making."
Rubio also said during a separate interview that even though he has yet to win an election, he doesn't think he's the second choice of voters.
"I think now that the race begins to narrow you're going to have a clear indication of it," he said on CBS's "Face the Nation."
"We feel good about the states we're going into and the process that they award delegates by. And we're in real good shape, we've got to keep working hard," he said.
--This report was updated at 11:17 a.m.