Missile defense advocates and critics alike are up in arms over the latest failed intercept test of the Ground-based Midcourse Defense (GMD) system, and well they should be: That’s three straight flops for the primary U.S. territorial missile shield.

The botched tests undermine the credibility of the GMD, which was hurriedly deployed in response to an emerging threat from North Korea. U.S. taxpayers have poured tens of billions of dollars into the system, which currently features 26 interceptors at Fort Greely, Alaska, and four at Vandenberg Air Force Base, Calif. The system is slated to undergo an expansion in the years ahead. The Pentagon plans to add 14 interceptors at Fort Greely, while congressional missile defense hawks are pushing for work to begin next year on a third U.S. interceptor site to counter Iran’s growing missile capabilities.

It’s too early to pinpoint the cause of the July 5 failure — an investigation by the Missile Defense Agency (MDA) likely will take weeks or months to complete. But a prominent group of Republican lawmakers wasted little time in identifying an underlying cause: budgetary neglect by the Obama administration.

In a July 12 letter to U.S. Defense Secretary Chuck Hagel, these lawmakers, who hold senior positions on the House and Senate Armed Services committees, noted that GMD funding has declined by half since 2008, the last budget crafted by President Barack Obama’s predecessor, George W. Bush. “Such funding cuts have touched every facet of the GMD program, including its maintenance,” the lawmakers said. 

Their case against Mr. Obama is unconvincing, however. While GMD funding indeed has fallen from about $2 billion in 2008 to roughly $1 billion today, a similar decline, albeit more gradual, was projected by the Bush administration. Mr. Obama got there more abruptly, in 2009, by canceling his predecessor’s plan to add 14 interceptors in Alaska and expand the system into Europe. One can debate the merits of those decisions — Mr. Obama obviously reversed course on the former — but testing, which is funded across several MDA accounts, is another matter. Speaking of which, the MDA conducted three GMD flight tests in 2010 — as many as were carried out in any year by the Bush administration. Two of those were attempted intercepts and both ended in failure, which helps explain the two-year hiatus in GMD intercept tests prior to July 5.

The failures do suggest — strongly — that the MDA and its GMD prime contractor, Boeing, have a quality control problem. The agency’s director, U.S. Navy Vice Adm. James Syring, told lawmakers that the quality and reliability of the GMD interceptors “must be our top concern” in light of the misfires.

It’s also quite plausible that GMD critics were right in saying the system was deployed before its time. Pushing unproven systems into production has been a longstanding complaint about the MDA leveled by, among others, the Government Accountability Office. 

One thing everybody can agree upon is that more GMD testing is essential. In their letter to Mr. Hagel, the Republican lawmakers called for a repeat of the failed test at the earliest possible date. No quarrel here: This is, after all, the system that America depends upon today for protection against a North Korean missile attack. A repeat attempt might not be feasible in 2013 — the MDA first must identify the root cause of the failure and fix it — but Congress should be prepared to appropriate the funding necessary to make it happen in 2014. Testing, and lots of it, should take priority over adding interceptors at Fort Greely or a third U.S. site — there’s little point in expanding the system before ensuring that it works.