For months, protesters have been rallying outside Senator Cory Gardner’s offices in Colorado, urging him not to join fellow Republicans in their push to repeal the Affordable Care Act. When he refused to hold town hall meetings, protesters staged them in his absence, asking questions to a cardboard cutout of him.

Now they are escalating their tactics.

With the controversial Republican health bill heading to the Senate, organizers of opposition groups say they plan to bird-dog Mr. Gardner’s whereabouts and show up at his events, buying seats at fund-raisers if necessary. They want to remind him that he promised to protect the expansion of Medicaid and guarantee coverage for people with pre-existing conditions, both provisions undone by the bill the House narrowly passed last week.

“I’m really angry right now,” Katie Farnan, an organizer of Indivisible Front Range Resistance, said. “It seems like they’re out to destroy the country.”

Grass-roots groups around the nation have pledged to confront their members of Congress more intensely than ever in the hope of stopping the bill in the Senate, where Republicans have expressed concern about the millions of Americans expected to lose health insurance coverage or benefits if the bill is passed in its current form. And they are looking ahead to the midterm elections next year, vowing to punish Republican House members who voted in favor of the American Health Care Act, the formal name for the bill.