The Maine senator who caucuses with Democrats told CNN's Erin Burnett on "OutFront" that his colleagues had made an unfair and ill-conceived decision to invoke a rarely used Senate rule barring Warren from debate over President Donald Trump's pick for attorney general, Jeff Sessions.
As for the question of whether or not Warren, a Massachusetts Democrat, was right to try reading a letter from Coretta Scott King criticizing Sessions, he called it a "close call."
King said the incident would likely lead to a fundraising bonanza for Democrats, saying he thought it was "a terrible mistake" by Republicans. He also said he understood accusations of sexism over the incident.
"I don't know if it (sexism) was directly the cause, but you can't avoid the conclusion," King said.
The Maine senator took issue in particular with Senate Republicans choosing this moment to invoke a rule against "impugning" another senator when other such moments have passed without consequence.
"What really bothered me about last night was the selective enforcement of this rule," King said. "I just don't think it was appropriate, particularly when it had been ignored in other situations fairly frequently since I've been around here."
The interview came just before the Senate confirmed Sessions to lead the Department of Justice. King has voted in favor of many of Trump's nominees, but he has also stood with Democrats in opposing some of them, like Education Secretary Betsy DeVos and Sessions.
"I'm taking them one at a time," King said.
Still, he predicted the Republican Party would stay unified and get through all of Trump's Cabinet choices.
"Will any of them be rejected? It doesn't look like it. It looks like the Republican 52-vote majority is going to stay right where it is," King said.
He noted a symbolic victories against DeVos, pointing out Vice President Mike Pence had to come in as a tie-breaking vote in the Senate.
"They had to bring in the vice president, first time in history, to take Betsy DeVos across the finish line. But I think as a practical matter, they have the votes and that's what's going to happen," King said.