As the struggle for control of the Senate played out in swing states across the country on Tuesday night, the Democrats were quickly assured of preserving one of their safest seats: Senator Chuck Schumer of New York, as expected, was re-elected to a fourth term — clearing the way for him to become his party’s leader in the Senate.

Mr. Schumer easily flicked away his Republican challenger, Wendy E. Long, who had eagerly embraced the outsider ethos and convention-defying policy positions of Donald J. Trump, the Republican nominee for president, riding New York’s Democratic majority and his own deep name recognition to a nearly effortless victory.

Mr. Schumer will succeed Senator Harry Reid of Nevada, who is retiring, as the chamber’s Democratic leader.

Mr. Schumer, a former congressman from Brooklyn who was first elected to the Senate in 1998, rumbled past Ms. Long, a Manhattan lawyer whose only other foray into electoral politics — as Senator Kirsten E. Gillibrand’s Republican opponent four years ago — also ended in double-digit defeat.