Yet as the president outlines his plan, his critics say he is trying to have it both ways. He is, they say, taking credit for a drawdown that has been envisioned since he first announced the current buildup on Jan. 10 — a withdrawal that had to be carried out unless he was willing to take the politically unpalatable step of extending soldiers’ tours further.

The White House declined on Wednesday to preview Mr. Bush’s speech, but one senior administration official, speaking anonymously to avoid upstaging the president, said the reductions would be heavily conditioned on the situation in Iraq and would fall far short of the rapid withdrawal Democrats want.

Under the plan, at least 130,000 American troops would remain in Iraq next July, down from more than 160,000, decreasing to about the same level as before the buildup began, with any decisions on further withdrawals likely to be postponed until at least next March. The planned drawdowns between now and July 2008 are expected to be of the 30,000 that many assumed the president would suggest after this week’s testimony by Gen. David H. Petraeus, the top military commander in Iraq. But, the senior official said, Mr. Bush’s ultimate goal would be a sustainable force of around 10 combat brigades, down from 20 now, at the end of his presidency, though a large number of support troops would also still be required.

“We want bipartisanship,” said this official, “but not to the point where it sacrifices success.”

Mr. Bush has repeatedly asked Americans to give him another chance in Iraq, and Thursday night will be no different. “His main goal at this critical juncture,” said another senior official, also speaking anonymously, “is to ask Americans to stop and take a fresh look.”

Whether they will take that look remains to be seen. This week’s Congressional testimony from General Petraeus was supposed to be a defining moment in Washington’s debate over the war.