“I’m not being naïve,” Mr. Manchin insisted, acknowledging that Senator Mitch McConnell of Kentucky, the Senate Republican leader, has vowed to block Mr. Biden’s agenda. “We’d be a lot better if we had participation, and we’re getting participation, but when it comes time to a final vote…” He trailed off.

He also suggested that Senate Democrats were partially responsible for the current dilemma on the filibuster in the Senate, noting that it was the majority leader at the time, Harry Reid of Nevada, who first removed parts of the filibuster in 2013.

“What goes around comes around here, they all understand that,” Mr. Manchin said. “And there were 33 Democrats in 2017 that signed a letter to ‘please save the filibuster and save our democracy.’ That’s what I’m trying to do.”

Democrats pushed back on that suggestion, saying the erosion of support for the filibuster on their side of the aisle stemmed from the abuse of the rule by Republicans. That was capped by a Republican filibuster late last month of a bipartisan commission to investigate the origins and implications of the Jan. 6 attack on the Capitol by Trump supporters.

Senate Democrats have already had passionate closed-door meetings about the voting rights bill, going state by state through restrictions on voting access either enacted already or making their way through Republican-held legislatures. So far, they have resisted breaking up the For the People Act and passing less partisan measures, like the ethics provisions meant to block profiteering off the presidency and opening the business interests of presidents and vice presidents to more public scrutiny.

Mr. Blumenthal said if Mr. Manchin is firm, conversations about legislative strategy will pick up steam. “We can’t let the perfect be the enemy of the good,” he said.

“These measures are about ending campaign finance corruption and political self-aggrandizement in a way that is fundamental to preserving our democracy, along with preserving access to the franchise, which is central to our democracy,” he added. “Maybe there will be choices ahead, but we need to be very careful about the sacrifices that could be made if we rethink too radically what For the People contains.”

Chris Cameron contributed reporting.