“Today what unites us is more important than what divides us,” he declared.

Skepticism abounds, and many details, including the division of staffing and budget for a unified conference, remain unresolved. The longtime chief deputy under Ms. Stewart-Cousins, Senator Michael N. Gianaris of Queens, who has clashed with Mr. Klein, is expected to remain in the leadership. In the past, Mr. Klein and Mr. Gianaris have also run rival Democratic political operations.

Still, the accord represents a watershed for fractious Democratic politics in the state and a political coup for Mr. Cuomo as he faces his most serious primary challenge. For years, Mr. Cuomo had said it was not in his power to arrange a “shotgun marriage” between Mr. Klein and Ms. Stewart-Cousins, but as a deal came together the governor was very much in the middle of it.

In a brief phone interview, Ms. Nixon said “Andrew Cuomo can’t have it both ways” — claiming he was powerless to corral the I.D.C. for years and then taking credit for it disbanding on Wednesday. She accused Mr. Cuomo of working from the “old playbook” of centrist politics.

“He is trying hard to evolve,” she said, “but I think it’s a little too late.”

Ms. Nixon noted the accord came less than a week after lawmakers struck a deal on a $168 billion budget, when the state’s priorities were set. Ms. Stewart-Cousins had been excluded from those discussions.

“I would have the leader of the Senate Democrats in the room negotiating,” she said.

Mr. Cuomo’s team touts his progressive record, which includes approving a minimum-wage increase, paid family leave and some of the tightest gun-control measures in the nation, despite a Republican State Senate.

Mr. Cuomo has burnished those credentials since Ms. Nixon emerged on the scene, and on Wednesday he personally called Bill Lipton, the state director of the liberal Working Families Party, to describe the Senate unity agreement, according to three people familiar with the call.

Mr. Cuomo is actively seeking the endorsement of the Working Families Party, which has its own ballot line in New York. He has also been calling other leaders across the state on the phone to sell the unity package as key to helping the Democrats win a Senate majority.