WASHINGTON — The bitter divide over the fate of the Affordable Care Act played out at the highest levels on Wednesday as President Barack Obama urged Democrats on Capitol Hill to fight to maintain his signature domestic achievement while President-elect Donald Trump warned Republicans to let Obama's imperiled program "fall of its own weight."

As the Senate began debate over the fate of the law, Republicans made clear, more powerfully and explicitly than before, their intention to repeal it. Vice President-elect Mike Pence, standing with Speaker Paul D. Ryan in the Capitol, said the repeal would be "the first order of business" for the new administration.

By a vote of 51-48, the Senate decided on Wednesday to take a first step toward repealing the Affordable Care Act. Senators agreed to take up a budget resolution, or blueprint, that would clear the way for legislation repealing major provisions of the health care law. Senate debate on the budget measure is expected to continue for several days.

The House is expected to take up a similar budget resolution next week.

Pence said Trump's transition team was working on a "series of executive orders that will enable that orderly transition to take place."

But Democrats vowed to strongly resist the repeal efforts, which Sen. Chuck Schumer of New York, the Democratic leader, said could be reduced to the slogan "make America sick again."

Schumer said Republicans would have to own the health care system. He said Democrats would consider working with Republicans on a replacement only after they presented their own plan.

"If you are repealing, show us what you'll replace it with, then we'll look at what you have and see what you can do," Schumer said.

Ryan tried to offer assurance that no change in coverage would be abrupt.