The Senate marked time in its Iraq debate on Friday, laying the rhetorical groundwork for a showdown next week over its latest effort to force President Bush to begin withdrawing U.S. troops.

During a morning debate that wandered off on a variety of unrelated topics, the Senate adopted, 87-1, an amendment to the fiscal 2008 defense authorization bill (HR 1585) to double to $50 million the bounty on Osama bin Laden.

Debate begins in earnest next week on more contentious amendments, starting with the principal Democratic challenge to Bush’s policy — an amendment by Armed Services Chairman Carl Levin, D-Mich., and Jack Reed, D-R.I., that would mandate a drawdown and redeployment of U.S. forces in Iraq by the end of April 2008.

On Thursday, the House passed by 223-201 a bill (HR 2956) that would require a similar redeployment by the end of March 2008, one month earlier than the Levin-Reed proposal.

The White House has threatened to veto any legislation that attempts to set a withdrawal timetable or dictate the mission of U.S. troops in Iraq.

Senate Republicans appear to have enough votes to block adoption of the Levin-Reed proposal. Sixty votes are needed to surmount a filibuster, and so far, there are not enough GOP defectors to allow Democrats to prevail.