D'Esposito conceded Thursday afternoon.
Gillen is a former town supervisor who campaigned on supporting law enforcement and border security, an issue many Democrats blame for their losses in New York in 2022.
Gillen was one of several Democrats running against a New York Republican congressman in a district that Democrat Joe Biden carried in 2020.
Gillen had also run against D'Esposito in 2022 but narrowly lost in a district just outside New York City that President Joe Biden had won by 15 points two years earlier.
This time, Gillen, who is an attorney, adjunct law professor and former Hempstead town supervisor, came away with a triumph in the Long Island suburbs. A year ago, the GOP held all four House seats representing the island. Now, each party holds two.
Gillen released a statement that she is ready to get work for her district:
"I am humbled and grateful to have the privilege of serving the people of New York's Fourth Congressional District. Voters chose our message of common sense and common ground, and that is exactly the approach I will bring to Washington. As Town Supervisor, I reached across the aisle to lower costs for Nassau County families, and in Congress I will work in a bipartisan manner to deliver on the pressing issues we face."
The district falls just outside the New York City borough of Queens.
D'Esposito released a statement thanking his volunteers and supporters:
"I am incredibly proud of the campaign, particularly in the face of a massive Democratic voter registration advantage, as well as the tens of millions of dollars Washington progressives spent on lies to smear me and my family and attack the values that I share with my Long Island neighbors. From the NYPD to Capitol Hill, I have always strived to serve the public, and I will continue doing so."
Two other Republican freshmen, besides D'Esposito, were also denied a second House term in New York.
Separately, ABC News projected Democratic candidate Josh Riley to win in New York's 19th Congressional District against Republican incumbent Marc Molinaro.
In 2022, Molinaro defeated Riley by less than three percentage points, and President Joe Biden won the district in 2020 by around five percentage points.
(The Associated Press contributed to this report.)
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