(Adds Trump tweets, Pence meeting lawmakers, AMA letter)
By Susan Cornwell and Richard Cowan
WASHINGTON, Jan 4 (Reuters) - Vice President-elect Mike Pence on Wednesday met with Republican congressional leaders to plot strategy on repealing President Barack Obama's signature healthcare law, a move that could leave tens of millions of Americans without medical insurance.
Obama on Wednesday morning was meeting with Democratic legislators, including U.S. Senate Democratic Leader Chuck Schumer, to discuss how they can protect the 2010 Affordable Care Act, which is known as Obamacare.
Republican U.S. President-elect Donald Trump, who takes office on Jan. 20, and his party's congressional leaders have made repealing and replacing the law among their top priorities. But doing so risks causing chaos in the health insurance market, as well as political backlash against Republicans.
In a series of posts on Twitter, Trump urged fellow Republicans to assign blame to Obama's Democrats.
The law has enabled upward of 20 million Americans who previously had no medical insurance to get coverage and is considered Obama's top legislative achievement. Republicans, who will control both Congress and the White House in 2017, condemn it as a government overreach.
"Republicans must be careful in that the Dems own the failed ObamaCare disaster, with its poor coverage and massive premium increases," Trump tweeted.
"Don't let the Schumer clowns out of this web," he added.
Obama walked into a Democratic caucus meeting on Capitol Hill accompanied by House Democratic leader Nancy Pelosi. He ignored a shouted question from a reporter about how to stop repeal of the healthcare law.
Schumer and Pelosi were to hold a news conference after the session.
Republican Representative Mark Amodei of Nevada said Pence had told Republican lawmakers that "we are mindful of disrupting the markets." Pence and House Republican leaders also were due to address reporters.
Trump has vowed to protect some popular parts of the Obamacare law, such as barring insurance companies from denying coverage to people with pre-existing medical conditions. But he wants to replace it with a system that is "much better and much less expensive," as he told Reuters on Oct. 25 after premium increases emerged in some healthcare markets.
Republican Representative Chris Collins, a liaison between the Trump transition team and Congress, said he did not expect Pence to have definite answers to detailed healthcare policy questions such as what timeframe should be considered for repealing the law.
The transition team, Collins said, is "not into that kind of meat and potatoes."
A House Republican leadership aide said there are lots of Republican "ideas" but it was too early to know what will end up in replacement legislation.
The American Medical Association doctors' group urged caution in making changes to Obamacare, which the organization supported.
"In considering opportunities to make coverage more affordable and accessible to all Americans, it is essential that gains in the number of Americans with health insurance coverage be maintained," the AMA said in a letter to congressional leaders.
The AMA said before any action is taken on Obamacare policymakers should lay out for the American people "in reasonable detail what will replace current policies."
"Patients and other stakeholders should be able to clearly compare current policy to new proposals so they can make informed decisions about whether it represents a step forward in the ongoing process of health reform," the group said. (Additional reporting by Mike Stone; Writing by Will Dunham and Steve Holland; Editing by Leslie Adler and Bill Trott)