WASHINGTON (Reuters) - President Barack Obama exhorted fellow Democrats on Wednesday to preserve his legacy-defining healthcare law as Republicans moved ahead with their long-sought bid to scrap it in what Vice President-elect Mike Pence called the “first order of business” of Donald Trump’s administration.

The emerging Democratic strategy is to warn that Republicans risk throwing the entire U.S. healthcare system into chaos by moving to dismantle the 2010 Affordable Care Act, popularly known as Obamacare, without a plan to replace it.

Republicans argue the system is already broken and that they will help more people gain coverage by repealing the law while working to minimize disruptions to those who depend on it.

Both Obama and Pence visited Capitol Hill for closed-door discussions on Obamacare.

Pence, the Indiana governor and a former member of the U.S. House of Representatives, met Republican lawmakers to plot the path forward on scuttling the law.

“The first order of business is to keep our promise to repeal Obamacare and replace it with the kind of healthcare reform that will lower the cost of health insurance without growing the size of government,” Pence told a news conference.

Down the hall from Pence, Obama, who hands over the presidency to Trump on Jan. 20, urged Democratic lawmakers to protect his signature domestic policy measure. He told reporters his message was: “Look out for the American people.”

Democrats acknowledge they lack the votes needed to stop repeal legislation being pushed by Republicans, who will control the White House and both chambers of Congress when Trump takes office. But they are warning of the risks of the repeal legislation in hopes of spurring a public backlash against it.

Without a replacement by Republicans, as early as 2018, the roughly 20 million people who gained insurance under the law could see their coverage in jeopardy.

“The Republican plan to cut healthcare wouldn’t ‘make America great again,’” Senate Democratic leader Chuck Schumer told reporters, invoking Trump’s campaign slogan. “It would make America sick again and lead to chaos instead of affordable care.”

Since the law was enacted, Republicans in Congress have voted more than 50 times to try to repeal all or part of it and conservatives have filed suits to try to invalidate it.

Republicans criticize Obamacare as an excessive government intrusion into the healthcare market and contend it is harming job growth by adding burdens on businesses.

REPUBLICANS ‘HAVE A PLAN’

Slideshow ( 10 images )

Republicans on Wednesday stepped up their rhetorical attack on Obamacare, with House Speaker Paul Ryan saying the law ruined the American healthcare system.

Trump wrote on Twitter that Republicans “must be careful in that the Dems own the failed ObamaCare disaster, with its poor coverage and massive premium increases.”

Pence said Trump would work with congressional leaders for a “smooth transition to a market-based healthcare reform system” through legislative and executive action.

Slideshow ( 10 images )

Republicans have offered few details, however, on what a replacement for Obamacare would look like. Ryan said lawmakers would take action that did not “pull the rug out from anybody” and that the party had “plenty of ideas.”

Democratic Senator Debbie Stabenow said Republicans were “pulling the string that’s going to unravel the whole (healthcare) system.”

Schumer said any effort by Republicans to shift the blame to Democrats would fail: “They want to repeal it and then try to hang it on us. Not going to happen. It’s their responsibility, plain and simple.”

The House Republican Study Committee, composed of the most conservative members of the chamber, signaled they were aware of the risk of appearing not to have a plan and filed a bill on Wednesday that could constitute a replacement.

House Republicans last year offered a proposal that would, among other things, provide refundable tax credits to help people afford their medical insurance premiums.

Obamacare helped people obtain insurance by increasing funding to states to expand the Medicaid healthcare program for the poor and providing government subsidies to help people obtain coverage from private insurers through government-run exchanges.

Republican Senator Rand Paul, who is a doctor, joined the Democrats in voting against beginning consideration of the Obamacare repeal resolution. While Paul said he supported repealing the law, he added the current proposal would increase the government debt by $9.7 trillion in the next 10 years.

“Is that really what the Republican Party represents?” Paul said during Senate floor debate.

The United States has a more complicated healthcare system than some other rich nations whose governments provide medical coverage.

Many Americans get health insurance through their employers. Others buy policies directly from private insurers or are eligible for government-run programs for elderly and low-income people. About 29 million had no medical insurance in 2015, according to the most recent government statistics. The U.S. population tops 320 million people.

Trump has vowed to protect some popular parts of Obamacare, such as barring insurance companies from denying coverage to people with pre-existing medical conditions.