Rep. Lauren Underwood (D-Ill.) speaks Tuesday during a news conference on Capitol Hill unveiling health-care legislation to shore up the Affordable Care Act. (Andrew Harrer/Bloomberg News)

House Democrats have spent several weeks battling over some of the most divisive policies, from support for Israel to combating climate change.

But no issue has the potential to sharply split the new majority quite like health care, with more than 100 of the caucus’s most liberal lawmakers advocating a sweeping proposal for universal coverage that would eventually eliminate the employer-provided insurance plans that cover 80 percent of Americans.

Medicare-for-all has also been dominating the nascent 2020 Democratic presidential campaign, serving as an ideological fault line to determine a candidate’s liberal bona fides.

Then came President Trump, acting White House chief of staff Mick Mulvaney and their decision Monday to push the Justice Department to argue in federal court that the entirety of the Affordable Care Act should be destroyed.

In a matter of days, Democrats circled their wagons and geared up for a fight to defend what they consider the most important piece of domestic policy legislation of this century — a law they successfully defended on Capitol Hill two years ago that propelled them into a midterm election landslide and the House majority in November.

“Okay, let’s go,” Rep. Lauren Underwood (D-Ill.) recalled thinking to herself. “We have go do it again.”

Rep. Katie Hill (D-Calif.) said any attack on the ACA, through legislation or the courts, was the single most unifying issue for House Democrats — from Rep. Alexandria Ocasio-Cortez (D-N.Y.), the liberal star of the freshman class, to Rep. Collin C. Peterson (D-Minn.), the 29-year veteran centrist from a rural district Trump won by more than 30 percentage points.

“Everybody agrees that the ACA is better than nothing,” Hill said. “You better believe not a single one of us is going to let that get overturned.”

Underwood and Hill are Exhibits A and B of the political potency of the health-care debate for Republicans, examples of why Senate Majority Leader Mitch McConnell (R-Ky.) and House Minority Leader Kevin McCarthy (R-Calif.) are so reluctant to follow Trump’s call to make the GOP the “party of health care.”

In 2017, Underwood, now 32, had never run for office but had served as a policy adviser at the Department of Health and Human Services during the implementation of the law commonly known as Obamacare. Back home in the exurbs of Chicago, Underwood contemplated running for office and, once her Republican congressman voted to repeal the ACA, she knew her next move.

“That’s how I decided to run,” she recalled.

Hill, 31, entered the race for the longtime Republican seat north of Los Angeles in March 2017. Also a first-time candidate, she had been working for local nonprofit agencies, which included helping California’s expansion of Medicaid that came through the ACA.

At a town hall a few weeks later, she challenged the GOP incumbent, Steve Knight, to explain how he would vote on the ACA repeal legislation. He declined to talk about the “hypothetical” bill, only to vote for it not long thereafter.

It provided the foundation for her campaign, including early advertising about her husband suffering a collapsed lung just before they wed. They ended up $200,000 in debt because he was switching jobs and lacked health insurance.

“That was the key driver for so many people from day one,” Hill recalled of her campaign.

Underwood and Hill won, both defeating veteran Republicans.

In a February conference call with donors this year, McCarthy pointed to the May 2017 vote as the moment the majority was basically lost. House conservatives rejected an early version of the repeal bill and negotiated a new plan that removed guaranteed protections for those with preexisting medical conditions and other critical benefits.

For the next 18 months, Democratic campaigns unloaded on Republicans.

“Basically, House Republicans handed Democrats a political club — and Democrats gleefully beat them over the head with it in the 2018 midterms,” said Michael Steel, a longtime House GOP strategist who advised the independent advertising unit for the National Republican Congressional Committee last year.

On Thursday, McCarthy hesitated when asked to describe a conversation he had with Trump about his “party of health care” declaration.

“We support preexisting conditions, we have been very clear on that, and that’s — a lot of our conversation was about that,” McCarthy said.

Privately, McCarthy had told Trump it was a bad idea. He recognizes that the GOP pledge to repeal and replace Obamacare is the political holy grail for Republicans, always just out of reach and ending in spectacular failure in its pursuit.

McConnell is leaving any new health-care proposal to White House officials. Instead, he wants to put the focus back on Medicare-for-all to divide Democrats and appeal to swing voters by saying Democrats would bankrupt the government with their far-left policies.

“I am focusing on stopping the Democrats’ ‘Medicare-for-none’ scheme,” he told Politico’s Burgess Everett.

Sen. Lindsey O. Graham (R-S.C.) spent last weekend with Trump and Mulvaney at a fundraiser for the senator and golfing at the president’s Mar-a-Lago resort. He encouraged the president to go big on health care, telling him 2018 was a political bloodbath because Republicans had no counter to Democrats on the issue.

“If we don’t have a proposal on health care, then that’s a mistake going into 2020,” Graham told reporters after Trump’s visit to the Senate GOP luncheon.

Trump and Mulvaney went further than just considering a new proposal — they forced the Justice Department to litigate against the entire ACA. That left all Republicans on the defensive on an issue they would rather just avoid.

Underwood called it another “immediate and very present threat” to the biggest unifying policy in the Democratic caucus. Before Trump’s move, Democrats had been in what she politely described as the “brainstorm phase” of how to tackle the debate between shoring up the existing health-care system or ripping it apart and starting over.

That debate, on Capitol Hill and on the presidential trail, has split Democrats apart. Then came the new assault on Obamacare, a law she likened to a friend battered and bruised after a bar fight. It’s someone everyone knew wasn’t perfect but was ready to defend.

“She’s still standing,” Underwood said.

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