WASHINGTON - Senate Republicans, scrambling to head off GOP defections to a resolution opposing President Bush's war policy, are considering their own resolution demanding benchmarks to measure progress in Iraq and possibly a new diplomatic effort to end the war, senators said yesterday.

Senators from both parties began preparing for a showdown with Bush over his plan to increase troop levels in Iraq, though that showdown may be pushed back to the week of Feb. 5. Two rival camps opposed to the additional troop deployments sparred over the wording of a resolution expressing the Senate's opposition, while GOP leaders and White House loyalists plotted a response.

Minority Whip Trent Lott (R., Miss.) conceded that his party's leadership could not head off a vote, as the White House wants. But GOP leaders will try to force a vote on a resolution that they hope will satisfy Republicans who are leery of the president's approach but are reluctant to oppose him.

"What we need is to put together a list of benchmarks that are tough and measurable, with requirements that the administration report back on the progress," said Sen. George Voinovich (R., Ohio), who has expressed deep concern with the Bush policy but voted Wednesday against a Foreign Relations Committee resolution of opposition.

Democrats said the GOP efforts might keep some, but not all, Senate Republicans from signing a resolution opposing the troop increase. At least a half-dozen Republicans have signed on to one of two nonbinding resolutions: the strongly worded measure that passed out of the Foreign Relations Committee and a more deferential version drafted by Sen. John W. Warner (R., Va.).

Foreign Relations Chairman Joseph R. Biden Jr. (D., Del.) and his resolution's coauthor, Sen. Chuck Hagel (R., Neb.), requested a meeting with Warner and his supporters to try to negotiate a single resolution that could attract broad support.

Warner turned down those overtures last night, saying that he would introduce his resolution on the floor as it is written and that any amendments would have to be made during open debate.

The two resolutions come to similar conclusions, opposing the introduction of 21,500 more troops, calling for more diplomacy and a regional peace effort, and demanding that U.S. troops be deployed away from urban sectarian hotbeds to guard Iraq's borders, hunt down terrorists, and train Iraqi security forces.

Warner's version starts with deferential language accepting Bush's constitutional powers as commander in chief, leaves rhetorical room for some additional troop deployments, and treats the fight with Sunni extremists in Anbar province as a matter separate from the sectarian violence in Baghdad.

While the lawmakers negotiate, antiwar groups are launching a public-relations blitz to sway Congress. A demonstration in Washington is planned for tomorrow. And a coalition of labor unions, liberal activists and Iraq war veterans, called Americans Against Escalation in Iraq, will barnstorm through Minnesota, Indiana, Ohio, Maine, New Hampshire, Pennsylvania and Virginia next week to try to pressure wavering Republican senators.

In each state, TV advertising will show six veterans, intoning: "When it comes to Iraq, America is divided. On the one hand, you've got two-thirds of the American people, a bipartisan majority in Congress, the Iraq Study Group and veterans like us, all opposed to the escalation."

Then a final veteran, missing one arm, concludes: "On the other hand, there's George Bush, who supports escalation. If you support escalation, you don't support the troops."