Rubio: It's getting harder to stick to pledge to support Republican nominee
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Republican presidential candidate Marco RubioMarco Antonio RubioOvernight Defense: Senate passes 0B defense bill | 3,000 US troops heading to Afghanistan | Two more Navy officials fired over ship collisions Senate passes 0B defense bill Trump bets base will stick with him on immigration 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 John TrumpBiden slams Trump over golf gif hitting Clinton Trump Jr. declines further Secret Service protection: report Report: Mueller warned Manafort to expect an indictment MORE.

"Because while I don’t want Hillary ClintonHillary Diane Rodham ClintonBiden slams Trump over golf gif hitting Clinton Overnight Cybersecurity: Equifax hit by earlier hack | What to know about Kaspersky controversy | Officials review EU-US privacy pact Overnight Tech: Equifax hit by earlier undisclosed hack | Facebook takes heat over Russian ads | Alt-right Twitter rival may lose domain 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."