President Trump plainly feels the nation’s grief and anger over young Nikolas Cruz’s shooting rampage at Marjory Stoneman Douglas HS. The question is: Will he seize the chance to do something about mass shootings?

As classes were ending Wednesday in Parkland, Fla., Cruz walked into the school that had expelled him, tripped a fire alarm . . . and started shooting as teens ran for their lives or hid.

The carnage left 17 dead and 14 more wounded, making it the deadliest school shooting since the 2012 Sandy Hook massacre, which claimed 26 lives, and the third worst after the 2007 attack at Virginia Tech.

And it came just five months after a much older shooter wreaked havoc at a concert in Las Vegas, slaughtering 59 innocent people and injuring hundreds more.

Trump on Thursday spoke to “a nation in grief,” promising “every parent, teacher and child who is hurting so badly” that “we are here for you, whatever you need, whatever we can do to ease your pain.”

Citing the kids “who were stolen from us,” lives of “unlimited potential” lost, the president said, “No child, no teacher should ever be in danger in an American school. No parent should ever have to fear for their sons and daughters when they kiss them goodbye in the morning.”

And: “We must actually make [a] difference.”

Absolutely, sir. But, sorry, a meeting with governors and state attorneys general to make school safety a top priority isn’t remotely enough.

No, you need to get behind some steps that can help rein in these endless nightmares. No, not end them altogether — but at least to limit the toll and reduce the frequency.

Trump ran for president promising to protect the Second Amendment and warning of Hillary Clinton “coming for your guns” if she won the White House. But just as a lifelong anti-Communist Richard Nixon went to China as president, Trump can use that credibility to push common-sense legislation and regulations that can have some effect:

Reinstate the federal assault-weapons ban, or at least revive its key features. Passed in 1994 but allowed to lapse 10 years later, that law prohibited the manufacture of semiautomatic firearms that bore certain features, like detachable magazines, that made them more dangerous. Perhaps most important, it also outlawed “large capacity” magazines. Critics argue that the ban did little good — but the fact is that the average toll from mass shootings has been growing. It’s surely worth trying to trim a casualty from the next killer’s total. Note, too, that the ban did no real harm. And it certainly didn’t lead the nation down the “slippery slope” toward eliminating other weapons, let alone a repeal of the Second Amendment, as the NRA and other Washington lobbyists warned.

Raise the age to buy firearms. While Nikolas Cruz’s background and motives are still being investigated, it’s already clear he had issues — and people knew it. But at 19, his record wasn’t enough to prevent a gun sale. Background checks are no good if you hardly have a background. Most states ban drinking under 21; there’s no reason not to similarly curb gun purchases. Ban gun sales to “fugitives from justice.” Such sales have long been illegal, but last year, Team Trump opted to exclude fugitives from the background-check database unless they crossed state lines; that removed 500,000 names from the list. Shouldn’t everyone who flees justice be kept from buying guns?

Target bump stocks. These let shooters turn semiautomatic weapons into ones that fire almost as rapidly as fully automatic ones, which are illegal. The idea of a ban got attention last year after the devices helped the Las Vegas shooter carry out his massacre — but soon died. Congress should act. If it won’t, Trump can order a bureaucratic ban: No one claims the Second Amendment protects these things.

Kill the Concealed Carry Reciprocity Act. This bill would effectively impose some states’ loose gun-control laws on states with tighter ones; it’s arrogant and anti-federalist. It’s probably going nowhere — but the president can send a clear message by denouncing it. Why mess with places like New York City, which is already the safest large city in America? No doubt other steps can help address the mass-shooting horror. We’re all for better reporting of threats and better security — but no one wants America’s schools turned into fortresses. Nothing can be sure to prevent another Parkland or Las Vegas or Sandy Hook. But that’s a poor excuse not to act. America needs to stand up to these attacks — to make it harder for those who shouldn’t have firearms to get them, to send a message that the nation is cracking down. If it’s possible to prevent or limit even a few tragedies, why not?

Mr. President, this is your moment. You can keep your promises to the kids and the parents and honor your offer to do “whatever we can do.”

Prove how much you truly want to curb the carnage — and refuse to play hostage to the extremists on either side of these issues.

You have the famous presidential bully pulpit — and you also have Twitter. You can keep your agenda front-and-center, no matter how much Congress wants to duck or dither.

It’s time, Mr. President, to do something.

America is waiting on you.