Republican Tom MacArthur, one of the wealthiest members of Congress, was supposed to glide to re-election this year without much trouble.

He has money to burn, Donald Trump won his district by 6 points, and his Democratic challenger is a newcomer to the game, Andy Kim.

But it's not going to script. MacArthur, R-3rd, is losing ground and is now only slightly favored by the most respected rating groups.

Inside Elections recently downgraded his prospects and called the district a "credible takeover opportunity" for Democrats. And the Cook Political Report is on the verge of calling it a toss-up.

"I didn't think it would develop into the competitive race it has become," says David Wasserman of Cook. "It looks now like somewhere between lean (Republican) and toss-up. I'm surprised I'm saying that."

The big issue: Health care.

MacArthur is the guy who rescued the Obamacare repeal in the House, a measure that would have left 24 million more Americans without coverage, while stripping away key protections for the elderly and those with pre-existing conditions, like cancer or diabetes.

He won huge praise from the Republican leadership for his role, and no doubt secured himself a pathway to power within the party. He didn't just support the repeal: He was the mediator who brought aboard the extremists of the Freedom Caucus, which he did by making the repeal even more harsh than it had been.

MacArthur drafted an amendment that helped break the deadlock. It allowed states to apply for waivers so that insurers could impose much higher premiums on those with pre-existing conditions. Think of it as added punishment for getting sick, a provision that lands especially hard on older patients.

"People here are furious about it," says Kim.

I wanted to talk this over with MacArthur, but he refused. Which gives me some taste of what his voters experience when he refuses to hold town hall meetings.

But a few weeks back, MacArthur sent a mailer to his voters, financed by taxpayers, that tells you all you need to know about his strategy. In it, he claimed that his amendment would "make the coverage of pre-existing conditions sacrosanct."

That's fake news, folks. The amendment does bar insurers from refusing coverage outright, but it allows them to hike premiums to ruinous levels so that few can afford it. So, really, what's the difference?

According to the CBO, "less healthy people would face extremely high premiums" under MacArthur's amendment, a conclusion echoed by a chorus of experts and organizations, including the American Medical Association.

Sacrosanct? Kim calls that a flat-out lie. I find it hard to argue the point.

Back to the election. MacArthur's district stretches across the belly of New Jersey, from just south of Trenton to Toms River. Parts are in Ocean County, a Republican stronghold that skews older, and parts are in Burlington County, which leans Democratic.

"Kim has a two-prong strategy here," says Wasserman, at Cook. "to energize the base voters in Burlington, but also to depress the traditional Republican vote in Ocean County by talking about the age tax and the Republican health care bill."

One reason the race has tightened is that Kim, a national security adviser under President Obama, has raised $2.2 million, more than expected. He's been frugal so far and had $1.7 million on hand, as of his most recent report.

Expect the bulk of that to be spent on TV ads reminding voters not just about MacArthur's amendment, but about his broken promises leading up to it.

Here's a list of whoppers from a rare town hall meeting MacArthur held in March of 2017, during the peak of the Obamacare debate:

"I don't introduce bills without a Democratic co-sponsor." The amendment had no Democratic co-sponsor, and not a single Democrat voted for the final bill.

"I have made it crystal clear that if we pull the rug out from under the most vulnerable, I can't support that." The repeal would have deprived 24 million of coverage, with low-wage families the hardest-hit of all.

"It has to cover pre-existing conditions. It has to cover that." As discussed, opening the door to big price hikes could put coverage out of reach.

Each of those lines drew big applause for MacArthur at his town hall. But Kim has enough money to air 30-second commercials that show that each claim was duplicitous.

"Democrats are going to spend their money reminding voters of that," says Nathan Gonzales of Inside Elections. "This district is going to be competitive. I think Kim has proven some fundraising mettle. He's not going to outspend the Congressman, but he'll raise enough to be credible. Republicans are in a precarious position."

New Jersey has 12 seats in Congress, and five of them held by Republicans. Democrats are favored to pick up two more seats this year, with state Sen. Jeff Van Drew in the 2nd, and former Navy helicopter pilot Mikie Sherrill in the 11th.

Among the three remaining Republicans, the most vulnerable is Rep. Leonard Lance, R-7th, according to the ratings agency, and the most secure is Rep. Chris Smith, R-4th.

MacArthur sits in the middle, still favored to win, but not by much. And losing steam, with more than three months to go.

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Tom Moran may be reached at tmoran@starledger.com or call (973) 836-4909. Follow him on Twitter @tomamoran. Find NJ.com Opinion on Facebook.