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Andy Kim, a New Jersey Democrat who had been locked in a neck-and-neck race for Congress, has defeated his Republican opponent, a two-term incumbent and staunch ally of President Trump, according to a final vote tally.

After a drawn-out vote count, Mr. Kim, a former national security adviser under President Barack Obama, was declared the winner by a slim margin, beating Representative Tom MacArthur, who wrote a key amendment in the effort to repeal the Affordable Care Act, according to The Associated Press.

The victory by Mr. Kim caps a devastating midterm election for Republicans in New Jersey.

With 11 Democrats now holding House seats, Representative Chris Smith is the lone Republican in Washington from New Jersey; the last time Democrats had such a large advantage Woodrow Wilson was in the Oval Office.