Sen. Bernie Sanders said Tuesday that he will not support Chuck Schumer’s bid to lead Senate Democrats next year, delivering a public challenge to the New York senator as the party prepares to take control of the chamber.

Sanders, an independent from Vermont who caucuses with Democrats, announced his decision alongside Sen. Elissa Slotkin (D-Mich.) after the party’s first full caucus meeting in Washington following sweeping midterm election victories.

“I personally have known Chuck Schumer for over 30 years, and I like Chuck. Nobody who works harder than Chuck Schumer,” Sanders said. “But in my view, the time is now for change, and I will not be supporting Chuck to be reelected as the Democratic leader.”

The announcement comes as Schumer faces pockets of opposition within his caucus and criticism from several outside advocacy groups. No other senator has formally entered the leadership race against him.

Slotkin stopped short of ruling out Schumer, but said she would not support any candidate until they presented a plan for the party’s priorities in the first 100 days of the new Congress.

Sanders said he and Slotkin had heard Schumer’s ideas in a serious discussion but found the proposal insufficiently defined. “Was it as definitive as it should be? In my view it was not,” he said.

Schumer expressed confidence Tuesday that he would win the leadership vote, pointing to shared priorities among Senate Democrats.

“I’m confident I will be elected majority leader,” he said. “And our caucus is really united on bringing down costs, on ending the corruption, on ending the chaos, on ending the war.”

Neither Sanders nor Slotkin identified an alternative candidate they would support. Both said Sen. Chris Van Hollen (D-Md.) was among the names being considered.

Slotkin described her Monday meeting with Van Hollen as productive, but said she would apply the same standard to every contender.

“I want to see what that plan is. I want to see how it represents newer members,” she said. “I will require that for anyone.”