Tom Suozzi, a former Democratic congressman, won a closely watched special House election in New York on Tuesday, narrowing the Republican majority in Washington and offering his party a potential playbook to run in key suburban swing areas in November.

His victory in the Queens and Long Island district avenged a year of humiliation unleashed by the seat’s former occupant, George Santos, and stanched a trend that had seen Republicans capture nearly every major election on Long Island since 2021.

Mr. Suozzi, 61, fended off the Republican nominee, Mazi Pilip, in a race that became an expensive preview of many of the fights expected to dominate November’s general election, especially over the influx of migrants at the border and in New York City.

A well-known centrist, Mr. Suozzi distanced himself from his party, calling for harsher policies at the border and vowing to work with Republicans to fix a broken immigration system. Polls suggested the independent approach helped narrow Ms. Pilip’s advantage on the issue, as Democratic super PACs deluged her with ads attacking her as anti-abortion.

In the end, the race also became an old-fashioned local contest over turnout as a rare Election Day snowstorm blanketed Long Island. The 11th-hour twist most likely helped Democrats, who had turned out in larger numbers during early voting despite Republicans’ vaunted Nassau County machine.

With 85 percent of votes counted, Mr. Suozzi had won 54 percent of the vote compared with 46 percent for Ms. Pilip, according to The Associated Press. The margin was expected to narrow as counting continues.