WASHINGTON — President-elect Donald J. Trump and congressional Republicans appear to have accomplished a feat that President Obama, with all the power at his disposal, could not in the past seven years: They have galvanized outspoken support for the Affordable Care Act.

People who benefit from the law are flooding Congress with testimonials. Angry consumers are confronting Republican lawmakers. And Democrats who saw the law as a political liability in recent elections have suddenly found their voice, proudly defending the law now that it is in trouble.

Thousands of people across the country held rallies over the weekend to save the health care law, which Republicans moved last week to repeal with a first but crucial legislative step. A widely circulated video showed Representative Mike Coffman, Republican of Colorado, eluding constituents who had wanted to meet with him to express their concerns on Saturday at a community event in Aurora, Colo. Rallies on Sunday to save the health law drew robust crowds around the country.

“We are here today — thousands strong in Boston, and at more rallies all across this country — because we will make our voices heard,” Senator Elizabeth Warren, Democrat of Massachusetts, told a crowd outside Faneuil Hall in Boston. “If Republicans try to rip health care out of the hands of millions of Americans, we will fight them every step of the way.”