There is no more perilous — or important — political activity than moving the “Overton window” of what constitutes an acceptable policy proposal. Sen. Bernie Sanders did just that by championing Medicare-for-all, and now his plan to nationalize health care has gone from the progressive fringe of the Democratic Party to the mainstream. So did Donald Trump by championing an “America First” foreign policy, and now he has made isolationism and protectionism mainstream in the Republican Party for the first time since the 1930s.

Democratic presidential candidate Beto O’Rourke is trying to shift the terms of debate on gun control by calling for a mandatory buyback of assault rifles. In last week’s Democratic debate, he recounted the devastating impact of a “high-velocity round” fired by one of these weapons (“when it hits your body, [it] shreds everything inside”) and then proclaimed, “Hell yes, we’re going to take your AR-15, your AK-47. We’re not going to allow it to be used against our fellow Americans anymore.”

Republicans immediately pounced. “The American people deserve to know this president, this vice president and these House Republicans will always stand for the Second Amendment right to keep and bear arms,” Vice President Pence thundered. A Republican state representative from the Houston area named Briscoe Cain even appeared to threaten O’Rourke by tweeting, “My AR is ready for you Robert Francis.”

While Republicans were predictably apoplectic, mainstream Democrats were nervous. Sen. Christopher A. Coons (D-Del.) warned, “That clip will be played for years at Second Amendment rallies with organizations that try to scare people by saying, ‘Democrats are coming for your guns.’  ” Presidential candidate Pete Buttigieg endorsed Coons’s criticism, suggesting reasonably enough that Democrats should focus on background checks, “red flag” laws and banning the sale of high-capacity magazines and new assault weapons — rather than trying to take away existing assault weapons.

O’Rourke was unrepentant. “Leaving millions of weapons of war on the streets because Trump and McConnell are ‘at least pretending to be open to reforms’?” he replied. “That calculation and fear is what got us here in the first place. Let’s have the courage to say what we believe and fight for it.”

O’Rourke has certainly shown courage on this issue. As a U.S. Senate candidate in Texas — Texas! — he advocated a ban on selling assault weapons. Now, following the massacre in his hometown, El Paso, he has been radicalized to go further. He is not, however, on a political kamikaze mission. Nearly 75 percent of Democrats support an assault-weapons buyback — and an even greater number support a ban on their sale. In a recent Post-ABC News poll, as my Post colleague Philip Bump points out, a buyback program was even backed by 31 percent of Republicans and 37 percent of people who live in households that have guns. By proposing a gun buyback, O’Rourke is breathing fresh life into his once-stagnant campaign.

That doesn’t negate the political risk of what O’Rourke is doing. His plan has the potential to mobilize gun owners and cost Democrats in the general election. But if so many people support a buyback even when so few political figures are advocating it (Democratic Rep. Eric Swalwell , of California, has been one of the few exceptions), imagine how popular opinion could shift if more leading Democrats got behind the idea.

It is certainly important to consider whether a policy idea is politically feasible at the moment — and that is a calculation that should be uppermost in the minds of legislative leaders. But it is even more important to consider what makes sense on the merits. And a mandatory gun buyback clearly does make sense. That’s exactly what Australia and New Zealand did after horrific mass shootings in their countries. In just six weeks, gun owners in New Zealand turned in 15,000 newly banned guns. In Australia, about 650,000 guns were turned in as part a mandatory buyback in 1996-1997. The result was a 42 percent decline in the rate of firearms homicides and a 57 percent decline in firearms suicides.

Buybacks are much more effective in reducing gun violence than simply a ban on sales of assault weapons of the kind that Congress instituted between 1994 and 2004. Millions of assault weapons have been sold since that law expired. To reduce the risk of mass shootings, it is imperative to reduce the number of weapons available for deranged individuals.

Second Amendment absolutists will scream that Americans have a right to own guns. That’s true — but there are limits. You can’t bring home a functional tank, a rocket-propelled grenade or a machine gun (at least not without a federal license). You shouldn’t be allowed to own a military-style assault rifle, either. It’s a weapon of war that has no place in civilian hands, and for all the political risks inherent in O’Rourke’s stance, I am glad to see a mandatory gun buyback entering the Overton window.

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