A memorial outside the Chabad of Poway synagogue in Poway, Calif., on Sunday. (Denis Poroy/AP)
Opinion writer

The Post reports:

On the final day of a major Jewish holiday, at a synagogue near San Diego, a 19-year-old man with an assault rifle and apparent anti-Semitic views opened fire, leaving one dead and three injured, authorities said. … Saturday’s shooting in Poway, about 20 miles north of San Diego, came six months after the massacre at Pittsburgh’s oldest synagogue, Tree of Life, where an armed shooter killed 11 people and injured six others in the deadliest attack on Jews in the history of the United States.

In an apparent manifesto posted online, the alleged shooter drew inspiration for his attack from that tragedy as well as the mosques shootings in New Zealand in March.

The Poway shooting occurred on the last day of Passover, a holiday celebrating Jewish freedom from persecution.

From these earlier attacks we know white nationalists are seeped in the toxic brew of anti-Semitism and “replacement" conspiracies. (”Jews will not replace us,” the Charlottesville neo-Nazis chanted) They tell us what animates their violence. The alleged murderer in Pittsburgh declared Jews were behind the plot to flood America with black and brown people, and the New Zealand shooter aligned himself with President Trump. The New Zealand and Pittsburgh massacres inspired the Poway attack.

Experts talk about “cascading terrorism” when one terrorist draws inspiration from prior acts. Clint Watts, a former FBI special agent and terrorism expert, observed that America “has serious domestic terrorism problem. Imagine any one of these shootings connected ISIS/[al-Qaeda], we’d be talking airstrikes, FBI task forces, [but] instead we get negligence under Trump.”

The murderer is legally and morally responsible for his heinous actions. Nevertheless, Trump’s sins of omission and commission cannot be ignored. He denies that white nationalism is an international, growing threat; he continues to focus on Islamic terrorism (e.g., the Muslim ban) to fire up his base although all domestic terrorist victims on his watch have been killed by white nationalists. His xenophobic and dehumanizing language about migrants that began with his campaign announcement in 2015 has never changed.

As Matthew Dowd, a former adviser to President George W. Bush, said on “This Week,” “I think the president needs to at some point look in the mirror and understand that the rhetoric, the words he uses in all of this, inflame a big part of what’s going on in America, give permission to the most craziest people in America, not that the president is responsible, but his rhetoric adds to that, and he needs to reflect on that.” The chances of that happening, however, are virtually nil.

Most recently, Trump praised himself at a rally for the “sick idea” of rounding up asylum seekers, as if they were cattle, and moving them to so-called sanctuary cities. When called out by former vice president Joe Biden, Trump decided to double down on his Charlottesville comments (“very fine people on both sides”), giving comfort and inspiration to white nationalists, a small number of whom commit these evil acts.

We don’t even have to imagine what Trump and his Republican enablers would say if President Barack Obama refused to identify the ideology of terrorists who are responsible for deaths on U.S. soil during his tenure. Trump and Republicans more generally excoriated Obama for failing to identify radical Islamic terrorism as the ideology behind their actions. He won’t even say the words. How can we defeat these monsters if we don’t identify who they are?

Rather than declare Biden’s focus on Charlottesville to be old news, maybe the other presidential candidates — and Congress, too — should speak with one voice. Trump may choose to wink and nod to white nationalists, but that should not mean our entire government is paralyzed.

Americans, regardless of party, need to denounce the president when he ignores white nationalism or tries to deflect attention with the preposterous claim that an entire party is anti-Semitic. Americans must demand that white-nationalist terrorism become a top concern and that efforts to combat it be fully funded. It is time for joint hearings on the issue, for social media to act (or face liability for material posted on the platforms) and for public leadership to re-marginalize the hate-mongers, who, thanks to the outrageous rhetoric on the right, have crawled out from the shadows to show off their ideology.

Republicans never seem to think loyal, law-abiding Muslims are doing enough to separate themselves from the violent extremists who falsely declare themselves defenders of the faith. (Not only do Muslim groups continually denounce terrorism, work in law enforcement and fight in the military, but they, of course, are also victims themselves.) So now it is Republicans’ turn to denounce conspiracy theories, throw characters such as Rep. Steve King (R-Iowa) out of their party and, yes, demand Trump denounce “white-nationalist terrorism.” Right now Trump seems content to play footsie with the fringe right, stoke fear of immigrants and traffic in conspiracy theories. He, quite simply, is part of the problem.

Related:

Read more:

Max Boot: After another synagogue shooting, we need better laws to fight domestic terrorists

The Post’s View: The U.S. needs to treat white supremacism as the worldwide killer it is

Brooke Binkowski: White supremacist violence has a long history in San Diego

Charles Lane: The fight against white supremacy could learn something from America’s first war on terror

Greg Sargent: Trump is likely emboldening hate groups. Time for tougher questions about it.