Congress opened for battle over the Affordable Care Act on Wednesday as Republicans pushed immediately forward to repeal the health care law and President Obama made a rare trip to Capitol Hill to defend it.
The bitterness that has long marked the fight intensified as Republicans seized the opportunity to make good on a central campaign promise to get rid of the law, a pledge reinforced on Wednesday by Vice President-elect Mike Pence, who met with House Republicans not far from where the president gathered with Democrats.
The Affordable Care Act, Mr. Obama’s signature health care law, has created online insurance marketplaces, offered new protections to people seeking health insurance, and provided coverage to millions of people near the poverty line through expanded Medicaid. Health policy experts say that system could collapse if Republicans cut off funds for the expanded coverage and end penalties for people who go without health insurance.
“The American people voted decisively for a better future for health care in this country,” Mr. Pence said, “and we are determined to give them that.” He said that President-elect Donald J. Trump would use his executive authority to help make the transition away from the health care law, but did not offer specifics.
Democrats vowed aggressive resistance, however, and said they would not participate in drawing up a replacement for the law after the swift efforts to unravel it. Senator Chuck Schumer of New York, the new Democratic leader, playing off Mr. Trump’s campaign slogan, said repealing the law would “make America sick again.”
Republicans are using a procedural approach that will allow them to repeal substantial parts of the health care law without Democrats’ being able to mount a filibuster in the Senate.
By a vote of 51 to 48 on Wednesday, the Senate took the first step, agreeing to take up a budget resolution, or blueprint, that would clear the way for legislation repealing major provisions of the law. But even as Republicans spoke of moving quickly to repeal the law, it remained far less clear how and when they would go about replacing it.
Senate debate on the budget resolution is expected to continue for several days, and the House plans to take up the measure once the Senate has approved it.
As Republicans charged ahead, both sides seemed cognizant of the possible fallout from unwinding the law, which has become deeply enmeshed with America’s health care system and has provided insurance for about 20 million people.
Mr. Trump weighed in with several Twitter posts. He advised that Republicans needed to “be careful in that the Dems own the failed Obamacare disaster,” and added, “Don’t let the Schumer clowns out of this web.”
Mr. Trump predicted that the health care law would “fall of its own weight.”
Representative Chris Collins of New York, a Republican who is one of Mr. Trump’s top supporters in Congress and is part of his transition team, said it was important to be sure that Democrats bear responsibility for the failings of the health care law. Republicans point out that premiums have risen and that consumers in many places have fewer choices of insurers.
“We have to make sure we keep reminding America, we are repealing it because it failed, we are repealing it because they all but demanded that we repeal it,” Mr. Collins said. “And that was a key piece of Donald Trump’s campaign.”
But as Republicans expressed eagerness to repeal the law, they acknowledged that replacing it would take more time. It is also unclear how insurance companies will react during this period and whether they will continue to offer the marketplace plans that millions of people have come to rely on.
“There will naturally be a reasonable transition period,” said Senator Ted Cruz, Republican of Texas. “You can’t adopt new reforms all at once.”
Senator John Cornyn of Texas, the No. 2 Senate Republican, noted that it had taken six years to get into “the ditch we find ourselves in now.”
“When your truck or car is in a ditch, the first thing you need to do is get out of the ditch,” Mr. Cornyn said. “And sometimes that takes a lot of hard work.”
To that, Senator Debbie Stabenow, Democrat of Michigan, parried that when a car goes into a ditch, “the first thing I don’t do is dismantle the car.”
“That doesn’t help me get anywhere in terms of transportation,” she said.
Democrats signaled little interest in helping Republicans determine what to do after repealing major parts of the health care law.
Mr. Schumer predicted that in a year, Republicans would “regret that they came out so fast out of the box.” He said Democrats would consider working on a replacement only after Republicans presented their own plan.
“If you are repealing, show us what you’ll replace it with first,” Mr. Schumer said. “Then we’ll look at what you have and see what we can do.”
Later, Mr. Schumer said of Mr. Trump, “It’s his and their responsibility, plain and simple — name calling isn’t going to get anything done.” He added, “They really need to calm things down a little.”
Speaker Paul D. Ryan tried to offer assurance that no change in coverage would be abrupt.
“The point is, in 2017, we don’t want people to be caught with nothing,” he said. “We want to make sure that there’s an orderly transition so that the rug is not pulled out from under the families who are currently struggling under Obamacare while we bring relief.”
Mr. Obama huddled with congressional Democrats for about 90 minutes in what was billed by the White House as a strategy session to forge a unified Democratic response to the Republicans’ rollback effort.
In reality, the session was essentially a going-away party for a man who passed his signature legislative accomplishment under long-extinct majorities in Congress and a “pep rally” for Affordable Care Act defenders, in the words of an attendee, Representative Hank Johnson, Democrat of Georgia.
The gathering, which could be Mr. Obama’s last trip to the halls of Congress that have been the site of alternating triumph and defeat, had a two-weeks-before-graduation air, with numerous Democratic lawmakers, including Mr. Johnson, sneaking out to attend to business more pressing than hearing the president’s words.
Mr. Obama, for his part, did not ask his allies to block all efforts to alter the law, but warned Democrats against “rescuing” Republicans by defecting on votes that would dismantle it.
The president provided an array of arguments for keeping the Affordable Care Act and offered a mild mea culpa for his shortcomings as a salesman over the years.
Senator Jeff Merkley, Democrat of Oregon, said, “He acknowledged the failures in selling the law in its entirety to the American people.”