WASHINGTON -- U.S. Rep. Tom MacArthur would seem an unlikely leader in the push to repeal and replace the Affordable Care Act.

A second-term lawmaker who co-chairs a group of Republican centrists, the South Jerseyan voted against the initial steps to repeal Obamacare in January and comes from a state where more than 500,000 benefited from the law's Medicaid expansion. Yet by Wednesday, he had emerged as a critical figure behind the revival of the stalled repeal plan, adding momentum to a major GOP pledge just ahead of President Trump's 100th day in office.

The move brought national prominence, and a bit of controversy, to a lawmaker still so new he is relegated to a cramped fifth-floor office.

The turnabout was only more stunning for the way he did it: winning a pledge of support Wednesday from the hard-right House Freedom Caucus, while alienating many of the moderates he is supposed to help lead.

"I have a choice," MacArthur said. "I can either be an obstructionist, or I can try to help make things better."

Still, the move gave new ammunition to MacArthur's critics on the left and threw an uncomfortable spotlight on his GOP colleagues from the Philadelphia region, who now face pressure to help pull the bill over the finish line. They barely held back their ire.

Rep. Charlie Dent, an Allentown Republican who co-chairs the same centrist group as MacArthur, called the process "terrible" and complained that his amendment "makes the whole situation worse, not better."

He said members of the 54-person Tuesday Group had urged their leaders not to negotiate with the conservative Republicans who make up the Freedom Caucus and had been the prime obstacle for a GOP repeal plan. MacArthur did it anyway.

Asked why, Dent said: "You'll have to talk to Tom."

MacArthur said that with Obamacare failing, he had to act to get a better bill over the finish line, regardless of whom he had to compromise with. "This bill is a lifeline to a health-care system that's in trouble," he said. ​

His plan would let states opt out of two popular Obamacare protections aimed at keeping insurance affordable for people with expensive medical needs and pre-existing conditions such as cancer, asthma, or pregnancies. That was a key change for conservatives who want less regulation and lower premiums.

The effort has encouraged the Trump administration and GOP leaders, who sounded ever more optimistic about dismantling Obamacare, perhaps in weeks.

"Certainly I can tell you that progress is being made and we're feeling good about that process," White House chief of staff Reince Priebus said this week.

He and other Republicans, however, stopped short of saying they had enough votes to pass the measure in the House. And MacArthur's plan could cost votes from moderate lawmakers who were already queasy about the bill and had new worries about undermining popular consumer protections.

Several House Republicans from the Philadelphia area, including Dent, South Jersey's Frank LoBiondo, and Bucks County's Brian Fitzpatrick, remain opposed to the measure. Lancaster County's Lloyd Smucker supported an earlier version of the bill; a spokesman said Wednesday he was reviewing the new version.

National Democrats cited MacArthur's plan as they launched a new wave of digital ads targeting him. He serves a swing district that backed Barack Obama in 2012 and President Trump last year.

MacArthur had already been on the hook for supporting the unpopular repeal plan that went nowhere. He was the only New Jerseyan to support the final Trump-backed proposal last month. Rather than let it die, he said, his idea would help lower costs and give states flexibility.

But liberal critics, independent analysts, and even some Republicans said the latest version would undermine protections for people with serious medical needs.

"This is the same bill that would cause 24 million Americans to lose health insurance. It's the same bill that will increase costs for everyone else," said Maura Collinsgru, health care program director for the liberal group New Jersey Citizen Action.

The repeal plan "was bad in March. It's *worse* now," the AARP wrote on Twitter. "We urge Congress to remain opposed ... and remember 38 million AARP members are watching."

MacArthur's proposal would let states easily obtain waivers for two Obamacare mandates, according to independent analysts and lawmakers.

One would let insurers out of a rule that bars them from charging higher rates to people with pre-existing medical conditions. The other requires insurance plans to include a set of "essential" benefits, such as coverage for maternity care and substance abuse.

The likely effect in states that received waivers: Costs for healthy people would drop, but those who need insurance most would face steep price increases, returning to discrepancies seen before the Affordable Care Act passed, said Larry Levitt, senior vice president of the Kaiser Family Foundation, a nonpartisan health policy group. Insurers, he added, could effectively charge  specific groups more.

One Kaiser study estimated that 27 percent of non-elderly adults have one of the pre-existing conditions that insurers once used to exclude them from coverage.  "While people with pre-existing conditions would theoretically be able to get insurance, there's no guarantee at all that the insurance would actually be affordable or have the benefits they need," Levitt said.

MacArthur said his bill would lower costs for many Americans, who would no longer subsidize people with expensive conditions, and would provide a safety net. States that scrap the mandates would have to create "high-risk pools" that cover people with costly medical needs. It would provide $130 billion to support those pools.

Levitt and others, however, said such pools had often proved ineffective.

Controversially, members of Congress would be exempt from the changes, though MacArthur said that was a technical problem due to Senate rules about the bill's language and would be addressed.

Dent worried that rank-and-file lawmakers could soon be forced to vote on MacArthur's plan even though it was unlikely to survive the Senate. "This is simply a matter of blame shifting and face-saving," he said.

Facing a crowd of reporters in his tight Capitol Hill office, MacArthur said he was focused on progress in the House.

He said, "I try to work on what I can control or influence."