On health care, it’s nice that Republicans have offered several alternatives to Mr. Obama’s government-heavy plan. But these alternatives have played no role in turning America against the president’s ideas. Opposition to ObamaCare in all its parts (not only its cost) has been the chief factor in flipping public opinion.

And that opposition has validated the noisy protests at Democratic town-hall meetings. Absent Republican opposition in Washington, the protests could be dismissed as insignificant. Together, congressional Republicans and their grassroots allies have become an influential force.

There’s an even more important consequence of Republican opposition. It’s preventing dozens of moderate House Democrats in Republican-leaning districts from going for ObamaCare. They won’t vote for it without Republican cover. Republicans are 40 votes short of a House majority, yet they’re thwarting Mr. Obama’s chief domestic priority. That’s effective opposition.

In one sense, Democrats have only themselves to blame. They set the standard for opposition during Mr. Bush’s second term. They furiously opposed everything with a Republican label on it. It worked so well that voters grew to prefer Democrats on practically every issue, including normally reliable Republican issues like taxes. A similar phenomenon is now benefiting Republicans.