President Barack Obama exhorted fellow Democrats on Wednesday to preserve his legacy-defining healthcare law as Republicans moved ahead with their long-desired bid to scrap it in what Vice President-elect Mike Pence called the "first order of business" of Donald Trump's administration.

The U.S. Senate brushed aside unified opposition by Democrats and voted to open debate on a resolution setting in motion the Republican drive to repeal the 2010 Affordable Care Act, which has helped upwards of 20 million previously uninsured Americans obtain medical insurance.

Obama, who hands over the presidency to Republican Trump on Jan. 20, made a rare trip to Capitol Hill to urge Democratic lawmakers to protect the measure, which is known as Obamacare and is considered his signature domestic policy accomplishment.

Republicans, who will control both chambers of Congress and the White House when Trump takes office, stepped up their rhetorical attack on the law, which they have labeled a government overreach. Democrats in turn accused them of trying to rip apart the nation's healthcare system with no firm plan to replace it.

"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, who have fought nearly all of Obama's major legislative initiatives during his eight years in office, have said Obamacare has brought excessive government intrusion into the healthcare market and contend it is harming job growth by adding burdens on businesses.

"LOOK OUT FOR THE AMERICAN PEOPLE"

Obama "encouraged us to fight," Democratic Representative Elijah Cummings told reporters after meeting with the president. Obama told reporters his message was, "Look out for the American people."

Pence, a former member of the U.S. House of Representatives, met Republican lawmakers to plot the path forward on scuttling the law. Afterward, they stepped up their rhetorical attack on Obamacare, with House Speaker Paul Ryan saying the law ruined the American healthcare system.

"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 said at a news conference.

Pence said Trump will work in concert with congressional leaders for a "smooth transition to a market-based healthcare reform system" through legislative and executive action.

During two news conferences, Pence, Ryan and Senate Majority Leader Mitch McConnell offered few details on what a Republican-backed replacement for Obamacare would look like. Ryan said lawmakers will take action that does not "pull the rug out from anybody" and that Republicans "have a plan" and "plenty of ideas."

"One of the most articulate Republicans on Capitol Hill is the speaker of the House," White House spokesman Josh Earnest said of Ryan. "And he did a news conference today where he was unable to explain why Republicans have not put forward their replacement plan." 

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

"Don't let the Schumer clowns out of this web," Trump added.

But Schumer said, "They want to repeal it and then try to hang it on us. Not going to happen. It's their responsibility, plain and simple."

"SMOOTH TRANSITION"

The Republicans risk causing turmoil in the health insurance market as well as potential political backlash if their policies fail. Pence said repealing Obamacare must be done in a way that does not inflict hardship on the economy or on Americans who obtained insurance through the law.

"The architecture of the replacement of Obamacare will come together, as it should, through the legislative process in the weeks and months ahead," Pence said.

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 joined the Democrats in voting against starting debate on the Obamacare repeal resolution.

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.