WASHINGTON, Feb. 5 — Republicans late this afternoon blocked a potentially momentous Senate debate, at least for now, on a bipartisan resolution opposing President Bush’s troop buildup in Iraq.

Forty-nine senators, almost all Democrats, voted to proceed with the debate, 11 short of the number needed under Senate rules on the issue. Forty-seven senators, nearly all Republicans, voted not to proceed.

This afternoon’s result cast doubt on whether the Senate would move toward a vote on what lawmakers of both parties described as the paramount issue of the day. Now it appears certain that more negotiations will take place on what war-related measure, if any, will be voted upon.

If 60 “yes” votes had materialized today, the Senate would have opened debate on a resolution sponsored by Senators John W. Warner, Republican of Virginia, and Carl Levin, Democrat of Michigan, that opposes the president’s plan to add about 21,500 troops to the American force in Iraq.