“It would be like the Dow Jones,” he said. “Nobody accuses the Dow Jones of being biased. It would be good information for all of us. And then you could say who’s winning and losing.”

So far, Republicans like Mr. Kingston are hanging with the president on the spending bill. They voted overwhelmingly against a measure, vetoed by Mr. Bush earlier this month, to set a timeline for troop withdrawal. But they must also worry about re-election in 2008 — a worry the president no longer has. Having already lost control of Congress, they can ill afford another election in which Iraq is the dominant issue. A standardized metric might give them a useful exit strategy.

But Michael O’Hanlon, the lead author of the Iraq Index, is skeptical. He says metrics are “important grist for a fact-based debate,” but history shows it is dangerous to rely on too few of them.

“Metrics were used in Vietnam, and we had the wrong ones, and in my opinion they did net harm to the debate,” Mr. O’Hanlon said, adding, “I’m afraid that Congressman Kingston is going to continue to be frustrated, because we can’t be exactly precise about which indicators are the conclusive ones.”

In any event, such an index would be politically unpalatable to the White House, which does not want to back itself into a corner by agreeing to someone else’s standard for progress. The White House says the only progress report that counts is the one from Gen. David H. Petraeus, the new top commander in Iraq, and Ryan Crocker, the new ambassador, who are expected to testify on Capitol Hill in September.

The two are apparently trying to prepare. They have spent the last month in Iraq consulting with a team of independent advisers who have been asked to “think through the question of, Is the current strategy for waging war going well or not?” said Stephen Biddle, a defense policy expert at the Council on Foreign Relations and member of the team. Mr. Biddle could not talk about his work, he said, but he did fault the White House for not being more open with the public about its own idea of what constitutes progress.

“By being unbelievably vague about everything,” he said, “they’re making it very hard for congressmen and senators to go to their constituents and say, ‘Look, here’s why things are going better than you might imagine.’ ”