In the heart of Long Island, where Republicans have won every major election in the last three years, Tom Suozzi fought through ripping political headwinds to claim victory on Tuesday in a special House election, seizing a coveted swing district that had been held by George Santos.

In the heart of Long Island, where Republicans have won every major election in the last three years, Tom Suozzi fought through ripping political headwinds to claim victory on Tuesday in a special House election, seizing a coveted swing district that had been held by George Santos.

Mr. Suozzi’s eight-point win flipped one of the five House seats Democrats need to retake the majority in November, giving the party a badly needed shot of optimism. But his campaign also provided something that may prove more valuable, a playbook for candidates across the country competing on turf where President Biden and his party remain deeply unpopular.