Sen. Bernie Sanders Bernie SandersButtigieg stands in as Pence for Harris's debate practice Bernie Sanders warns of 'nightmare scenario' if Trump refuses election results Harris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda MORE (I-Vt.) on Sunday criticized 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 for dismissing a recent CIA assessment that concluded Russia intervened in the U.S. presidential election to help the billionaire win the presidency.

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"This is very serious stuff. I think we go forward. But I don't want to go backwards," Sanders said Sunday on CBS's "Face The Nation."

"We've got to take a hard look at the role that the Russians played in this election process. We'll see where the investigation goes. But for Donald Trump to summarily dismiss all of this makes no sense to me at all."

The Washington Post reported on Friday that a secret CIA assessment concluded Russia intervened in the presidential election to help Trump win the White House.

Trump in an interview that aired Sunday blasted the recent report, saying he thinks it's "ridiculous" and "just another excuse."

"I don't believe it," the president-elect said in an interview that aired Sunday on "Fox News Sunday."

"I don't know why and I think it's just — you know, they talked about all sorts of things. Every week it's another excuse."

Sanders said on Sunday he is focused on making sure the president-elect fulfills the promises he made during the campaign.