WASHINGTON — As more than 40 subdued Republican senators lunched on Chick-fil-A at a closed-door session last week, Senator Cory Gardner of Colorado painted a dire picture for his colleagues. Campaign fund-raising was drying up, he said, because of widespread disappointment among donors over the inability of the Republican Senate to repeal the Affordable Care Act or do much of anything else.

Mr. Gardner is in charge of his party’s midterm re-election push, and he warned that donors of all stripes were refusing to contribute another penny until the struggling majority produced some concrete results.

“Donors are furious,” one person knowledgeable about the private meeting quoted Mr. Gardner as saying. “We haven’t kept our promise.”

The backlash from big donors as well as the grass roots panicked Senate Republicans and was part of the motivation behind the sudden zeal to take one last crack at repealing the health care law before the end of the month. That effort faltered Friday with new opposition from Senator John McCain of Arizona, the perennial maverick who had scuttled the Senate’s first repeal effort. Now Republicans must confront the possibility that they will once again let down their backers with no big win in sight.