Tom Suozzi, a Democrat, won the New York congressional seat vacated by the disgraced Republican George Santos on Tuesday night, in a boost for Joe Biden ahead of the presidential election.

In what had become an increasingly bitter campaign, the inexperienced Republican candidate, Mazi Pilip, attempted to tie Suozzi to the immigration situation at the US-Mexico border.

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The seat, in Long Island, was seen as a key indicator of voter sentiment before the expected Joe Biden-Donald Trump election in November. Biden won the district in 2020, but the area swung Republican in the 2022 mid-term elections, when George Santos was elected.

Santos was expelled from Congress in December after he was charged with more than 20 counts of fraud, sparking a special election. Even before the charges Santos had proved an intense source of embarrassment for Republicans, after it emerged he had fabricated huge chunks of his personal history.

Suozzi, who previously spent six years in the House of Representatives before quitting to run, unsuccessfully, for New York governor, will have to run again for the seat in the nationwide congressional elections in November.

The demographic of New York’s third congressional district had made this a closely watched election nationwide. The district, seen as a political bellwether, is largely suburban and was one of 18 districts that Biden won in 2020, but voted for a Republican House representative in 2022.

Immigration, abortion and aid to Israel featured heavily in both Suozzi and Pilip’s election campaigns, issues which are likely to remain important later this year.

Pilip, a relatively unknown local politician who was criticized for avoiding the press during the campaign, sought to tie Suozzi to Biden, whose approval rating remains historically low, on border issues, claiming the pair had “created the migrant crisis”.

Suozzi tried to distance himself from the left of the Democratic party by promising to “battle” progressive members of Congress. He accused Pilip of being anti-abortion – Pilip said she is “pro-life”, but would not support a national abortion ban.

Both Pilip, an Orthodox Jew who was born in Ethiopia before moving to Israel and who served in the Israel Defense Forces before coming to the US, and Suozzi are fervent supporters of continued aid, which became a key issue in a district which the Jewish Democratic Council of America estimates has one of the largest Jewish populations of anywhere in the country.

Suozzi’s victory further narrows the Republican party’s majority in the House of Representatives, where the GOP has 219 seats to the Democrat’s 213 – with three seats vacant – as Democrat’s hope to flip the House in November.