While it is far too soon to suggest that the House Republican majority may be imperiled, Democrats are newly optimistic about picking up seats in 2018, hoping to ride a backlash against Mr. Trump. Seeing an opportunity, they say they will not throw Mr. Trump a political life preserver at what they sense could be the first turns of a downward spiral.

The president’s approval rating was already mired below 40 percent in some surveys, and is likely to remain low after the health bill’s failure. He has no prospects for legislative victories on the immediate horizon, given how complicated and time-consuming his next priority, an overhaul of the tax code, would be even for a more unified party…

Though the ability of Democrats to do much more than say no remains limited, their success in helping to thwart Mr. Trump will not only embolden them to confront him again — it will also inspire activists to push them to do whatever it takes to block his path.

“Having tasted victory, the resistance forces will feel even more empowered to insist that Democrats continue withholding any cooperation and not granting Trump any victories when he is so wounded,” said Brian Fallon, a Democratic strategist.