WASHINGTON — A year ago Friday, Representative Claudia Tenney of New York stood among dozens of enthusiastic colleagues in the Rose Garden to celebrate passage of a bill to repeal and replace the Affordable Care Act. When President Trump made his way onstage, Ms. Tenney clapped and smiled.

On this not-so-happy anniversary, a Democratic “super PAC” is on the air with a television commercial reminding her constituents of a repeal vote that Republicans were once convinced would be a political winner. Ms. Tenney’s Democratic challenger, Anthony Brindisi, a state assemblyman, said health care is consistently one of the top issues in a vast district that runs from Lake Ontario through Utica and Rome to the Pennsylvania border.

“I think her vote is wildly unpopular among constituents in the district,” Mr. Brindisi said. “And what adds insult to injury is that she celebrated her vote by snapping selfies of herself at the White House, gleefully cheering a vote that would take health insurance away from thousands of her constituents.”

House passage of the Obamacare repeal bill left that chamber’s Republicans in a no-win situation. They took the hard vote, but because the Senate failed to follow suit, no one can claim a victory. Now Democrats hope to extract a price. Far from the liability that the Affordable Care Act has been in past elections, Democrats believe health care will be a key advantage heading into this fall’s midterm elections.