It used to be that a sworn oath to preserve, protect and defend the Constitution was the only promise required to become president. But that no longer seems to be enough for a growing number of Republican interest groups, who are demanding that presidential candidates sign pledges shackling them to the corners of conservative ideology. Many candidates are going along, and each pledge they sign undermines the basic principle of democratic government built on compromise and negotiation.

Both parties have long had litmus tests on issues — abortion, taxation, the environment, the social safety net. The hope was that the candidates would keep their promises, and, when they didn’t, voters who cared deeply about those issues could always pick someone else next time. Human beings, after all, do not come with warranties.

But iron-clad promises were just what the most rigid Republican ideologues wanted. They had seen too many presidents — specifically Ronald Reagan and George H. W. Bush — bend when confronted by a complex national reality. Both those presidents agreed to new taxes and some Republicans said they did not fight hard enough to outlaw abortion or cut spending to the point where government was unrecognizable. In other words, they compromised a bit, to keep divided government from destroying itself. Washington, the ideologues decided, corrupted true conservatives into moderates.

More was needed to keep them in line, which gave birth to the signed pledge — no more enforceable than a spoken promise, but a politician’s actual signature was seen as more binding. The oldest and most pernicious of these modern oaths was dreamed up by Grover Norquist, the leader of Americans for Tax Reform, who has managed to get 95 percent of all Republicans in Congress to pledge never to raise taxes for any reason. If they end tax deductions, Mr. Norquist’s pledge-takers say they will match the increase in revenue with further tax cuts.