Victories often contain the seeds of future defeats. So it is — or at least should be — with the Senate’s morally reprehensible rejection of expanded background checks for gun buyers.

The outcome is a test of both an invigorated gun safety movement and a gun lobby that decided to go for broke.

The National Rifle Association assumed that blocking new gun legislation in the wake of the Sandy Hook massacre would firmly establish its dominance. Advocates of sane gun regulations would scatter in despair and be torn apart by recriminations.

But there is a flaw in the gun lobbyists’ calculation: Their strategy leaves the initiative entirely in the hands of their opponents. The early evidence is that rage over the cowardly capitulation of so many senators to raw political power is pushing activists against gun violence to redouble their efforts.

What was striking about Wednesday’s vote is that many of the senators who had expressed support for universal background checks after the slaughter at Newtown meekly abandoned their position when the roll was called.