WILLINGBORO, N.J. — A year and a half ago, people worried about family members with pre-existing health conditions screamed at Rep. Tom MacArthur at a town hall here.

The New Jersey Republican didn’t just vote for his party’s health care plan, which had passed the House the week before. He was one of its authors.

A former insurance executive who moved to the 3rd District to run for Congress in 2014, MacArthur was long thought to be untouchable. Before brokering a deal with conservatives on health care, he’d been a leader in the Tuesday Group of moderate GOP lawmakers. He could afford to spend millions of his own money to defend a south Jersey seat covered by two pricey media markets. And he’d won re-election by 20 points in 2016, while this swing district backed President Donald Trump by 6.

MacArthur was an early target of national Democrats, but he seemed to begin the cycle in good shape for a GOP incumbent trying to survive a midterm referendum on the president.

That’s hardly the case anymore.