DOYLESTOWN, Pa. — Five years ago, the Affordable Care Act had yet to begin its expansion of health insurance to millions of Americans, but Jeff Brahin was already stewing about it.

“It’s going to cost a fortune,” he said in an interview at the time.

This week, as Republican efforts to repeal the law known as Obamacare appeared all but dead, Mr. Brahin, a 58-year-old lawyer and self-described fiscal hawk, said his feelings had evolved.

“As much as I was against it,” he said, “at this point I’m against the repeal.”

“Now that you’ve insured an additional 20 million people, you can’t just take the insurance away from these people,” he added. “It’s just not the right thing to do.”

As Mr. Brahin goes, so goes the nation.

When President Trump was elected, his party’s long-cherished goal of dismantling the Affordable Care Act seemed all but assured. But eight months later, Republicans seem to have done what the Democrats who passed the law never could: make it popular among a majority of Americans.