Sen. John McCain John Sidney McCainCindy McCain endorses Biden: He's only candidate 'who stands up for our values' Biden says Cindy McCain will endorse him Biden's six best bets in 2016 Trump states MORE (R-Ariz.) says he's worried Donald Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE could jeopardize Republican control of the Senate if he's the party's 2016 presidential nominee.

"Obviously we all know from history that if you have a weak top of the ticket, that has a significant effect on the states, particularly the swing states," McCain said at a Christian Science Monitor breakfast on Wednesday.

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"So, of course I worry. All of us have to worry about the viability of the top of the ticket," said McCain, who is up for reelection.

"We're a long way from next November," he cautioned. "And so we'll have to see how it all sorts out."

McCain, as he has in the past, stressed that he would support the Republican nominee, even Trump or Sen. Ted Cruz (R-Texas), whom he's clashed with in the past.

"I obviously disagree with Mr. Trump on certain issues, but I think that fight can be had within the Republican Party," he said. "We'll just have to see what happens, but I will support the nominee of the Republican party."

But McCain did have a warning for Trump, who has in the past threatened a third-party run if he doesn't seal the GOP nomination.

"You cannot alienate Hispanic voters and expect to win an election," he said.

McCain has given his full backing to fellow national security hawk and friend Sen. Lindsey Graham (R-S.C.) for president but weighed in on other candidates as well.

He said he admires New Jersey Gov. Chris Christie's "gumption," former Florida Gov. Jeb Bush's ideas, Ohio Gov. John Kasich's accomplishments and sees Sen. Marco Rubio (R-Fla.) in the "next generation of leaders in the Republican party."

McCain said he had strong philosophical differences about the role of government with Sen. Bernie Sanders (I-Vt.), who is seeking the Democratic nomination, but called him an "honest man."

"He's an honest man and his word is good. Once we reached an agreement, that agreement stuck," McCain said.

"And now he's brushing his hair, which is really remarkable," he joked.