WASHINGTON — Five weeks after President Bush won unfettered funding for America to fight the war in Iraq until September, Senate Democrats will try again on Monday to end the war.

Their opening gambit this summer will be three key amendments to the 2008 bill that authorizes defense spending. The bill is set to be marked up on July 9. Most appealing to wavering Republicans will be an amendment offered by a senator who served as President Reagan’s secretary of the Navy, a Democrat from Virginia, James Webb. Mr. Webb proposes to lengthen the leave time of active and reserve duty soldiers, making reinforcement of the current surge next to impossible.

Also offering amendments are Senator Feingold of Wisconsin and Senator Levin of Michigan. Mr. Levin, who chairs the Senate Armed Services Committee, would mandate troop withdrawals within four months of the law’s passage unless the Iraqi government meets political and security benchmarks. The bill allows Mr. Bush waiver authority. Mr. Feingold’s amendment requires the president to withdraw nearly all of the troops in Iraq by the middle of the presidential election season, April 2008.

“We are burning out our ground forces,” Mr. Webb said at a press conference alongside Democratic governor of Kansas, Kathleen Sebelius, this week. “If we’re honest about wanting to support our troops, there’s no better place to start than to correct our troop-rotation policy.”

Although Marine General Peter Pace, chairman of the Joint Chiefs of Staff, stated earlier this year that, for most active-duty troops, the “rotation has often gone to one year overseas, one year home,” many troops end up serving for up to 15 months at a time in Iraq, with only 12 months out of the country after that. The Pentagon extended active duty tours for the Army in April.

Mr. Webb’s amendment comes in the wake of proposals, put forth earlier this year by Secretary of Defense Gates, that went even further. The plan, unveiled in January 2007, required the leave-deployment ratio for active-duty members to be two years to one, whereas for National Guard members and Reservists, the ratio would be 5:1.

Pentagon officials say, however, that it is difficult to comply with these types of standards. As an alternative, the Pentagon is examining proposals that would offer compensation and incentives for soldiers serving longer tours.

In order to successfully overturn a likely presidential veto of any effort that would withhold funding or mandate a retreat from Iraq, Senate Democrats will have to convince a third of the Republican caucus to vote with them.

This will require Democrats “continuing to punch, day in and day out,” said Senator Biden of Delaware.

In a teleconference last week, President and CEO of the Center for American Progress, John Podesta, discussed efforts to sway Congressional Republicans on the war for Iraq.

“You just listen to the voice on the Republican side,” Mr. Podesta said. “They are nervous and looking for alternatives.”

The top Republican on the Senate Armed Services Committee, a critic of the administration’s strategy in Iraq, John Warner, echoed this sentiment.

Mr. Warner said that when the Senate reconvenes July 9, “You’ll be hearing a number of statements from other colleagues.”