WASHINGTON (CNN) -- Leaders of the "Out of Iraq" House caucus have circulated a letter to rank and file Democrats urging them to vote to fully fund the withdrawal of US troops from Iraq and to do so according to a "clear timeline."

A Democratic congressional source tells CNN anti-war Democrats are debating whether that timeline would include a six month or a year deadline for US troops to leave Iraq.

In the letter, the leaders of this caucus, including California's Barbara Lee and Maxine Waters, attempt to rebut the argument put forward by Republicans that "Congressional action on Iraq either constitutes micromanagement or cutting off funding for troops in the field."

"Fully funding withdrawal is not cutting off funding - we are going to fully fund a rational alternative to the administration's attempt to run out the clock on their failed policy," they write in the letter.

Two months after Democrats assumed control of Congress -- with what they said was a mandate from the American people to change course in Iraq -- Democrats in both the House and Senate are struggling to craft acceptable binding legislation to force the Bush administration to bring US troops home.

In the House, Speaker Nancy Pelosi is trying to bridge significant differences of opinion and strategy between a growing number of members of the "Out of Iraq" and "Progressive Caucuses" -- many of whom are refusing to vote on a war supplemental under any circumstances -- and other conservative Democrats from red states who worry that to put conditions on funding the war could tie the hands of US commanders on the ground.

Pelosi and other Democratic leaders are trying to find common ground with Congresswomen Lee and Waters who have signaled to Pelosi that unless they are given a vote on an amendment to fully fund a withdrawal by a date certain, then their membership would vote against the supplemental.