The nation’s fledgling missile defense system suffered its third straight test failure when an interceptor rocket failed to launch Sunday night from its base on an island, leaving the target rocket to splash into the Pacific Ocean, the Pentagon said Monday.

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President Bush pledged during the 2000 campaign to work for the deployment of a streamlined system suitable for the era after the cold war, and he has pushed to make it operational even as tests are being carried out, an approach that has prompted heavy criticism, especially with the recent failures.

“It’s as if Henry Ford started up his automobile production line and began selling cars without ever taking one for a test drive,” said David Wright of the Union of Concerned Scientists, which describes itself as a nonprofit organization dedicated to rigorous scientific analysis.

Mr. Wright, a co-director of the organization’s global-security program, said it was “irresponsible and potentially dangerous” for the Pentagon to assert that the emerging missile system is a reliable defense.

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The first group of land- and sea-based missiles and their associated systems will cost more than $7 billion. The overall missile defense program is expected to cost more than $50 billion over the next five years.