Sen. Bernie Sanders said President Trump "sold the American people out" last week in Helsinki, Finland, during his summit with Russian President Vladimir Putin.

The Vermont senator made the remarks in a joint interview with breakout Democratic Party star and candidate for Congress Alexandria Ocasio-Cortez. The interview will air Sunday morning on CBS News' "Face the Nation."

Asked whether Mr. Trump should withdraw an invitation to Putin for a second meeting in Washington this fall, Sanders said it's "hard to comment on anything that Trump says because he could change his mind tomorrow."

"But I will tell you that I was absolutely outraged by his behavior in Helsinki, where he really sold the American people out," Sanders told "Face the Nation" moderator Margaret Brennan. "And it makes me think that either Trump doesn't understand what Russia has done, not only to our elections, but to cyberattacks against all parts of our infrastructure. Either he doesn't understand it, or perhaps he is being blackmailed by Russia because they may have compromising information about him or perhaps also, you have a president who really does have strong authoritarian tendencies and maybe he admires the kind of government that Putin is running in Russia."

"And I think all of that is a disgrace and a disservice to the American people," Sanders said. "And we have got to make sure that Russia does not interfere, not only in our elections, but in other aspects of our lives."

During a joint news conference in Helsinki with Putin, Mr. Trump said he didn't see any reason why it "would" be Russia that was responsible for meddling in the 2016 election. Mr. Trump made the comment just three days after Deputy Attorney General Rod Rosenstein announced the indictment of 12 Russians accused of hacking in the 2016 election. Mr. Trump's performance during the press conference with Putin was widely criticized, including by many members of his own party.

Mr. Trump later said in a Cabinet meeting, in a reversal, "the sentence should have been, I don't see any reason why it wouldn't be Russia."

In that same Cabinet meeting, Mr. Trump also appeared to say "no" when a reporter asked if he believed Russia is still interfering. White House press secretary Sarah Sanders claimed the president was saying "no" to more questions, although exactly what Mr. Trump meant is still being questioned by some.

The confusion came as Director of National Intelligence Dan Coats said this week that Russia is engaged in "ongoing, pervasive efforts to undermine our democracy."

CBS News' Brennan asked Sanders how his campaign and party can protect themselves against foreign meddling.

"Margaret, that's a great question and I don't think anyone knows all of the answers, but one thing we do know is that we need a president who is going to do everything to work with statewide officials all over this country to make sure that when people cast a vote in November that vote is going to count," Sanders said. "Congress has allocated money to beef up and strengthen the protection of our electoral system. The president has got to be aggressive in implementing that.

"And by the way, another area of concern about Trump is when he was asked, 'Do you think that Russians are still meddling still interfering in our processes,' he said, 'No,' despite what Dan Coats suggested."

"So we have a lot of work to do, but the integrity of American democracy is at stake," Sanders said. "And we have got to do everything that we can to protect the integrity of our elections."

Sanders said his office will take any good advice they can get and already has spent time and money — as have other offices — to "protect the integrity of our information."

The full interview with Sanders and Ocasio-Cortez will air on "Face the Nation" Sunday at 10:30 a.m. ET. You can catch a rebroadcast of "Face the Nation" at 11 a.m and 6 p.m. ET on CBSN.