By Stephen Collinson

Washington, D.C. -- Democratic leaders of Congress Friday took the wraps off a new effort to force troop withdrawals from Iraq, hoping to heap more political misery on a weakened President George W. Bush.

The new assault on White House Iraq strategy follows the collapse of a landmark immigration drive on Thursday in the Senate, which dealt a grievous blow to Bush's hopes of a legacy-enhancing second-term domestic achievement.

Mounting Republican anxiety over the war is emboldening Democrats, who have been repeatedly frustrated by Bush's congressional allies in their attempts to handcuff him over Iraq policy.

'We have many arrows in our quiver, and we are sharpening them,' said House of Representatives Speaker Nancy Pelosi Friday, as members of Congress left town for a week-long Independence Day recess.

Pelosi said she would introduce a bill in July to authorize troop redeployments to start within four months and to be completed by April 1, 2008, a formula that Bush has already blocked once with a presidential veto.

The only exceptions would cover forces fighting terrorism and protecting US installations in the war-ravaged country, she said.

Senate Democrats will introduce their own attempts to force Bush to accept troop withdrawal timelines, extend rest periods for troops between deployments and curtail his congressional authorization to wage war.

'People in Iraq are celebrating dying of natural causes,' Reid said Friday, relating details of what he said was a 'mind boggling' radio report.

'If you die of a heart attack or cancer in Iraq, they celebrate because so many people are being killed prematurely ... I feel that I -- and just not as a senator, but as a person -- have a moral obligation to do what I can to stop the death and suffering. By pushing a withdrawal date, it does just that.'

The new Democratic initiative comes a month after party leaders bowed to Bush's demands to strip troop withdrawal timelines out of a new 100 billion dollar emergency war budget.

Although Democrats grabbed control of Congress in elections last November, partly on a wave of voter anger over the war, they lack the crucial super-majority of at least 60 votes in the Senate needed to overcome Republican blocking tactics.

Asked whether signs of ebbing Republican support for Bush could help the Democrats piece together 60 votes, Reid said : 'I don't know if we can or not. We're going to keep pushing, because it's the right thing to do.'

Bush was able to keep his firewall intact in the last showdown over Iraq because most Republicans and many Democrats were unwilling to take the ultimate step of halting funding for the war while troops were in harm's way.

But as the next Democratic assault will be attached to bills setting policy for the Pentagon, rather than to appropriation bills funding the war effort, some observers believe Republican support may be more prone to fracturing.

Bush on Thursday pleaded with Americans to give time for his strategy of surging nearly 30,000 extra troops into Iraq, to work, after the US death toll rose above 3,500 in the war.

'Americans have got to understand it takes a while to mobilize additional troops and move from the United States to Iraq,' he said.

Bush's plea came after two Republican senators broke ranks with renewed calls Wednesday for changes in US

Senator Richard Lugar, a reluctant rebel and foreign policy expert, sent shockwaves through Washington by warning the 'surge' would not work.

Fellow Senate Republican George Voinovich, who like Lugar has resisted Democratic attempts to curtail Bush's war powers, recommended a disengagement from Iraq.

The new Democratic attempt to skewer Bush on Iraq is seen as a prelude to what is shaping up as a crucial month in September, when the US commander in Iraq, General David Petraeus is expected to report on the progress of the surge.

Senior Republicans in the House and the Senate have said they expect Bush will have little option but to look for a course change in Iraq once the report is delivered.