For the second time in four months, the Senate on Wednesday blocked a bill to require establishing a national defense against long-range missiles as soon as the technology is available.

Republicans have made the speedy deployment of an anti-missile system a top national-security priority this year and tried to use the issue in the November elections to paint Democrats as soft on national defense.Democrats oppose the system because, they say, it is based on antiquated technology and would undermine international weapons treaties.

The 59-41 vote was mostly along party lines. While it was enough to approve the measure, it was one vote shy of the 60 needed under Senate rules to end a Democratic filibuster. No senator switched sides from a vote on the same bill in May, and Wednesday's outcome most likely closes the issue in the Senate this year, although the House will probably vote on a similar bill this month.