Mrs. Clinton said her push for a new vote on the war authority did not mean she would oppose whatever new spending measure might emerge from negotiations between Congress and the White House. But she said she was joining Mr. Byrd in trying to force a new examination of the war in its entirety, rather than simply joust over specific elements of the spending measure.

Talking to reporters after her floor speech in a mostly empty Senate chamber, Mrs. Clinton indicated that her view was that rescinding the original vote would mean that troops would be out as of October. “They have no authority to continue,” she said. “That is the point.”

Later, however, her aides said Mrs. Clinton was not seeking a total withdrawal of troops from Iraq, or a quick pullout that could put troops at risk. They said she had called for a phased pullout that would leave a reduced American force to pursue terrorist cells in Iraq, support the Kurds and conduct other missions — a position she continued to support, her aides said.

The White House spokeswoman, Dana Perino, said the Clinton-Byrd proposal represented the same sort of artificial timeline that led Mr. Bush to veto the $124 billion spending bill on Tuesday.

“Here we go again,” she said. “The Senate is trying another way to put a surrender date on the calendar. Welcome to politics ’08-style.”

The idea of revoking authority for the war has circulated on Capitol Hill for weeks without gaining much ground. Senator John W. Warner, Republican of Virginia, had raised the idea because the original resolution did not envision the prospect of troops caught in a civil war.

Clinton aides said Thursday that while Mr. Byrd’s advisers had talked to them this winter about withdrawing authorization, the new plan only came up between the two senators and their staffs earlier in the day. Moreover, one adviser to Mrs. Clinton said, President Bush’s veto of the Iraq spending bill had left her believing that new types of pressure were needed to force the White House to adopt an exit strategy.