Democratic-backed legislation to withdraw U.S. combat troops from Iraq cleared its first Senate hurdle Wednesday, but Republicans confidently predicted they would soon defeat it and President Bush backed them up with a veto threat.

The legislation, calling for combat troops to return home over the next 12 months, “would hobble American commanders in the field and substantially endanger America’s strategic objective of a unified federal democratic Iraq,” the White House said in a statement.

The strong veto message underscored the intensifying struggle between the administration and the new Democratic-controlled Congress and came on a day in which the Pentagon conceded in a report that “some elements of the situation in Iraq are properly descriptive of a civil war.”

President Bush and other administration officials have avoided saying that U.S. troops had been thrust into the midst in a civil war among Iraqis. By whatever name, the war has so far has claimed the lives of more than 3,100 U.S. troops, cost more than $300 billion and propelled congressional Democrats to power in last fall’s elections.

Democrats in the House and Senate are advancing different bills calling for the withdrawal of troops. Bush has threatened to veto both.

In the House, Democratic leaders said they were building support behind legislation to require the withdrawal of troops by Sept. 1, 2008, if not sooner. That plan faces its first test vote today in the Appropriations Committee, and Democrats circulated a letter of support from retired Gen. Wesley Clark, the former NATO commander who ran for president in 2004.

In contrast with the House bill, the Senate measure lacks a firm deadline for an end to U.S. participation in combat. It says a withdrawal should begin within 120 days “with the goal of redeploying, by March 31, 2008, all United States combat forces from Iraq except” for those needed for non-combat roles.

The only Iraq-related vote of the day was an 89-9 procedural roll call that cleared the way for a formal debate on the legislation in the Senate.

Senate Republican Leader Mitch McConnell said passage of the withdrawal measure “would be absolutely fatal to our mission in Iraq” – and he sought to rebut Democratic supporters with their own words.

Despite the legislation’s dim prospects, Democrats insisted the developments marked progress toward their eventual goal of forcing the president to end U.S. participation in the war.