Hillary Clinton’s former 2016 campaign manager Robby Mook said Monday that Russian interference in the election might have caused their team’s loss last November.

In an interview on “CBS This Morning,” Mook warned that a foreign government’s interference in the election hurt Democrats last year and it could happen to Republicans next time.

“I don’t think this is a partisan issue, I really don’t,” he said. “The idea that a foreign country has been able to get — to burrow into our political system and favor one candidate over another should be disturbing to everybody.”

When asked if he believes Russian interference caused Clinton’s loss, Mook responded, “I think it could have.” He pointed out he did not bring that up in the interview and was not trying to deflect responsibility for the Democrats’ loss. “This is the key, no campaign manager or candidate should ever sit at a table like this on a news program and say, ‘I don’t know, the Russian might have been the reason we lost the election.’”

“It’s not important whether they were or not,” he added. “What’s important is that they could have been the reason and that should never happen again.”

Mook said the public needs to know more about alleged conversations between Trump campaign officials and Russian intelligence before the election.

There are “two big missing pieces,” in his view. “We need to know more about what these conversations were with the Russians and Donald Trump’s staff, [and] we’ve got to see Donald Trump’s taxes,” Mook said, adding that he thinks they would show that Russia is deeply invested in Trump. “Congress has got to demand these taxes.”

Later in the interview, Mook compared Russian interference with the Watergate scandal, and accused Mr. Trump of lying about it.

“Look, somebody broke into the DNC [Democratic National Committee]. They didn’t do it physically this time, they did it digitally. It was a foreign country. The president has been lying about it. He lied about whether his staff were talking to Russia. The more we learn, the closer it gets to the president himself and the scarier it gets,” he said.