Hillary Clinton Hillary Diane Rodham ClintonFox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio Trump, Biden court Black business owners in final election sprint The power of incumbency: How Trump is using the Oval Office to win reelection MORE said Sunday the late Sen. John McCain John Sidney McCainAnalysis: Biden victory, Democratic sweep would bring biggest boost to economy The Memo: Trump's strengths complicate election picture Mark Kelly: Arizona Senate race winner should be sworn in 'promptly' MORE (R-Ariz.) "really understood in the marrow of his bones what it meant to be an American."

The 2016 Democratic presidential nominee said on NBC's "Meet the Press" with Chuck Todd that McCain knew how important it was for politicians to work together, and to trust each other.

"He knew that the Senate couldn't work if we didn't work together," she said.

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"I think it was heartbreaking to him that — as he said in the speech he gave right before he voted against repealing the Affordable Care Act, that we need to cooperate. We need to learn how to trust each other again and do better to serve the people who elected us," she added.

McCain died of brain cancer Saturday.

Clinton also said U.S. institutions, including McCain's beloved Senate, are being severely tested.

"And he was, in every way he knew how, trying to sound the alarm to get all of us as Americans to understand that if we abandon the ideals that we have stood for around the globe, if we turn our back on leadership on behalf of human rights and the kind of future we want to forge for our children and grandchildren, we will be giving up on what he fought for, what he was imprisoned for, what he stood for, and in a long line of American patriots," she said.