Senator Andrea Stewart-Cousins was poised to become the majority leader, making her the first woman to lead either legislative chamber in New York and the first black woman to lead the State Senate

Democrats seized control of the New York State Senate for just the third time in 50 years on Tuesday, a victory that could fundamentally alter the state’s economic and political fabric next year and beyond.

The Senate had been the Republican Party’s last foothold of power in an increasingly blue state. But after a closely watched, expensive battle, Democrats won eight Republican-held seats, giving their party decisive command of Albany’s triumvirate of power and positioning them to unleash a cascade of long-stymied progressive legislation.

Democrats had needed to flip only one seat to erase the Republicans’ razor-thin majority. They blew past that number, unseating five incumbents and winning three open seats.

“Thank you for sending us our biggest majority ever,” Senator Andrea Stewart-Cousins, the leader of the Senate Democrats, told supporters on Long Island.

Ms. Stewart-Cousins is expected to become the Senate majority leader, making her the first woman to lead either legislative chamber and the first black woman to lead the Senate.

“You keep making history, and you’ve made history for us,” she said.

Senator John J. Flanagan of Long Island, the leader of the Senate Republicans, called the outcome “disappointing” but promised to preserve Republicans’ voice in Albany.

“This election is over, but our mission is not,” he said in a statement.

The Democrats had campaigned on a decidedly Washington-focused message, latching on to the language of a national “blue wave” and promising to strike back at the Trump administration.