New Zealand banned semi-automatic rifles this past week and made the ban effective immediately to prevent any "stockpiling of weapons while the legislation is being drafted." The Kiwis are apparently not big on the hopes-and-prayers mantra the U.S. Congress adopts after each gun massacre.

It's easy to be dispirited by the cravenness of conservative politicians and by the frequency of American gun deaths. Yet those failures obscure a key development in U.S. gun politics: These are glory days for the gun-safety movement.

The March for Our Lives, which emerged from the 2018 massacre in Parkland, Fla., galvanized youth and inspired millions of Americans to march for stronger gun laws. In just six years, Moms Demand Action for Gun Sense in America has grown into a formidable political force, with millions of supporters, 350,000 donors and networked activists in every state. They are winning the argument: By about a two-to-one margin, Americans support stricter gun laws over leaving current laws in place. (Fewer than one in 10 Americans supports less strict laws.)

Emerging power has produced tangible results. According to the Giffords Law Center to Prevent Gun Violence, "The gun safety movement experienced a tectonic shift in 2018." The center tallied at least 67 new gun-safety bills signed into law last year.

That momentum continues. In Washington, the House of Representatives, flush with new Democratic members who campaigned on gun regulation, passed a universal background check bill in February. Multiple inquiries into the National Rifle Association's political activities and Russia connections are under way in Congress, with the NRA clearly on the defensive.

Meanwhile, state governments led by Democrats continue to adopt aggressive legislation to regulate firearms. For example, the state of New York in January moved to prohibit teachers from carrying guns in schools, extended the waiting period for gun buyers who don't pass an instant background check, and established a red-flag law to enable authorities to deny gun purchases by people who pose a danger to themselves or others.

It's been an extraordinary run. It may soon come crashing to a halt.

All it takes to turn back a robust gun-safety movement of millions is five willing justices. Or maybe just one. In the fall, the Supreme Court will hear arguments over an obscure and idiosyncratic New York City law that regulates the transportation of guns. It's possible the court will use that opportunity to declare a sweeping constitutional right to carry a firearm outside the home - one of many issues still unsettled after the court's 2008 Heller ruling, which by a 5-4 vote established a limited, but vaguely so, individual right to bear arms.

"The energy of the gun violence prevention movement can be sapped pretty quickly," said UCLA law professor Adam Winkler, author of "Gunfight: The Battle Over the Right to Bear Arms in America."

The court made seismic pronouncements on guns in 2008 and 2010, establishing first the individual right to firearms and then requiring states to recognize that right via the 14th Amendment. Then the court beat a retreat, declining to further define the scope of the Second Amendment.

Can semi-automatic rifles be banned? Does the Second Amendment guarantee the right to carry a firearm in a public place? The court opted not to clarify such questions. It may now, removing gun regulation from the heated political arena and declaring victory - or perhaps a series of victories - for gun-rights forces.

Brett Kavanaugh, who replaced Anthony Kennedy on the court, appears to be a maximalist on gun rights. As a circuit court judge he issued a dissent in a gun-rights case that was so sweeping and uncompromising that it could've been written at NRA headquarters. The other conservative justices appear likely to join him on the crusade. The main question concerns Chief Justice John Roberts.

In a concurring opinion in a 2017 ruling that upheld Maryland's ban on assault weapons, Judge J. Harvie Wilkinson of the U.S. Court of Appeals for the Fourth Circuit laid out the conservative case for restraint in gun cases. The courts shouldn't walk all over the political process, Wilkinson wrote.

Disenfranchising Americans on this would be the gravest and most serious of steps. It is their community, not ours. It is their safety, not ours. It is their lives, not ours. To say in the wake of so many mass shootings in so many localities across this country that the people themselves are now to be rendered newly powerless, that all they can do is stand by and watch as federal courts design their destiny, would deliver a body blow to democracy.

Francis Wilkinson writes editorials on politics and U.S. domestic policy for Bloomberg Opinion.