One of the most important authors of the GOP's ObamaCare repeal bill just went down in defeat

Democrats are continuing to add to their new majority in the House of Representatives — the party just flipped a seat held by a Republican who was key in the GOP effort to repeal ObamaCare.

Andy Kim, who served as a national security aide to former President Barack Obama, defeated two-term Republican Rep. Tom MacArthur (R-N.J.) in the race for New Jersey's 3rd District, The Associated Press projected on Wednesday.

Prior to the election, this race had been classified as a toss-up by the Cook Political Report. Kim declared victory on Election Night, but MacArthur did not concede the race, per Asbury Park Press. Elected in 2014, MacArthur was the architect of the controversial MacArthur amendment, a part of the GOP's proposed ObamaCare repeal which would have allowed states to opt out of some of the health care law's requirements.

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Since Election Day, Democrats have continued to pull off victories in key House races that had remained undecided last week. In New Jersey, in particular, they have made significant gains, as this is the fourth House seat the Democrats have won from the GOP in the state this year, reports The Hill. Kim's victory means that in the 116th Congress, New Jersey will have just a single Republican representative for the first time since former President Theodore Roosevelt's administration, The New York Times reports.