In recent years, the State Senate has been at the center of an epic — and often convoluted — struggle between Democrats and Republicans.

With the Assembly and governor’s office in Democratic hands, the Senate has been under Republican control, as a result of a power-sharing arrangement between eight breakaway Democrats, known as the Independent Democratic Conference, and the Republicans. A ninth Democratic senator, Simcha Felder of Brooklyn, also caucused with Republicans.

But with Cynthia Nixon’s energetic primary challenge of Gov. Andrew M. Cuomo, and the emergence of anti-Trump activists targeting state and local politicians in New York, the pressure had mounted on the I.D.C. to return to the mainstream Democrats. In early April, Mr. Cuomo helped broker a reconciliation between the two factions.

On Tuesday, Democrats in New York won special elections for two vacated Senate seats, giving them, in theory, a one-vote majority in the 63-seat body. But hours before polls closed that day, Mr. Felder declared that he was going to continue to sit with the Republicans — keeping that party in power.