WASHINGTON, Nov. 16 — Senate Republicans today easily blocked an effort by Democrats to act on a war spending bill that would have provided $50 billion for operations in Iraq and Afghanistan, but would have required that troop withdrawals from Iraq begin within 30 days.

The bill had numerous other strings attached a well, including a goal of completing re-deployment from Iraq by mid-December 2008 and a narrowing of the Iraq mission to focus on counter-terrorism and training of Iraqi security forces.

The vote today was the latest attempt by Democrats to force President Bush to shift his war strategy, but they were able to muster only 53 of the 60 votes needed to cut off debate and move to a vote; 45 senators voted against the measure.

A Republican counter-measure, which would have provided $70 billion for the military operations with no strings attached, also failed.