Rubio: It's getting harder to stick to pledge to support Republican nominee
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Republican presidential candidate Marco RubioMarco RubioRepublicans giving Univision the cold shoulder: report Week ahead: Senate panel to vote on Trump's Labor pick Senators introduce new Iran sanctions MORE said on Sunday that it's getting more difficult for him to stick to the pledge he made to support the eventual Republican nominee.

"Look, I’ll be honest with you: It’s getting harder every day. It really is," the Florida senator said on ABC's "This Week" Sunday, referring to supporting front-runner Donald TrumpDonald TrumpHouse Dems launch pro-broadband privacy petition Moving the point of obligation is the latest misguided ploy to undermine the RFS 'Morning Joe' co-host: We got into Trump's head MORE.

"Because while I don’t want Hillary ClintonHillary Rodham ClintonApril Ryan: 'I was in shock' Is America's military effort in the Middle East constitutional? Spicer: Trump will 'help the team' if needed in Georgia special election MORE to be president of the United States — I do not, I want her to be defeated — I think we’re having a battle to define conservatism in the Republican Party. I do not want the Republican Party or the conservative movement to be defined by what I’m seeing out of Donald Trump’s campaign."

Rubio said he knows people are angry and frustrated, but those in positions of leadership need to show these people "a way forward that gives them hope and a belief that we can make things better.

"That's real leadership," he said.

"That’s not what we’re getting from the front-runner," he added.

Rubio said he isn't going to change his position about supporting the nominee because he still believes Trump will not secure the nomination.

"I’m going to do everything possible to keep that from happening and to give the party a choice in me," he said, "someone that people aren’t going to have to be asked that question about."