Four of the Saturday worshippers brutally murdered at the Tree of Life synagogue had been old enough during the Holocaust and the second world war to revere America as a safe haven in a world of hate. The other seven victims, the youngest of whom was 54, had grown up in a land where most blatant antisemitism had seemingly retreated to the shadows.

When pipe bombs were mailed to a dozen targets of Donald Trump’s wrath, the only frail consolation had been that no one had been injured. Now with the worst incident of antisemitic violence in American history all comfort has vanished.

The sad undeniable truth is that this nation faces a grave challenge from homegrown terrorism that has nothing to do with Isis, al-Qaida or the Islamic religion. Most of these domestic terrorists lurk in the darkest corners of the crazed, conspiratorial far right wing. But the would-be killer who shot up a Republican congressional baseball practice in 2017 had been a supporter of Bernie Sanders.

Racial violence has never been far from the surface of American life as is hauntingly commemorated in Montgomery, Alabama, by the new outdoor memorial to the thousands of victims of lynching in the south. The 2015 murder of nine black parishioners in a Bible study class at a church in Charleston, South Carolina, serves as a reminder – like Pittsburgh – that even worshipping God offers no guaranteed safety in contemporary America.

The worst modern manifestation of domestic terrorism was the 1995 bombing of a federal office building, the Murrah Building, in downtown Oklahoma City. The mass murderers – raging against the government over a deadly 1993 confrontation in Waco, Texas – used a homemade fertilizer bomb to destroy the building. Another powerful outdoor memorial (which is tragically becoming an all too frequent architectural motif) depicts 168 chairs, one for each of the dead, including 19 children in a daycare center on the site.

In his 2004 autobiography, My Life, Bill Clinton notes that initially the speculation had been that “Islamic militants” were responsible for the Oklahoma City bombing. Instead, Clinton wrote hopefully, the realization that rightwing political rhetoric had become weaponized in Oklahoma City “prompted millions of Americans to reassess their own words and attitudes towards government and toward people who views differed from their own”.

If only that had been true.

The ugliness of political discourse, particularly on the right, both predated and powered the rise of Trump. But never, since the worst days of Joseph McCarthy, has anyone in a prominent public position matched Trump in his flagrant, continual use of guttersnipe rhetoric.

There have been hints that the accused pipe-bomb maker and the alleged murderer in Squirrel Hill may have been partly aroused by Trump’s demonization of the caravan of poor, desperate refugees from Central America seeking asylum. Roughly a thousand miles from the US border, the caravan is much further away than the frozen southern shores of Hudson Bay in Canada are from Detroit.

All it took to spread irrational fear was for Trump to tweet, without a shred of evidence, that “criminals and Unknown Middle Easterners are mixed in” the caravan. Today, 17 long years after September 11, no political image remains as powerful as the notion of swarthy Muslim fanatics in suicide vests sneaking across the border.

No similar mental pictures conjure up domestic terrorists. They walk among us like ordinary white Americans, perhaps more down on their luck than most and, yes, more prone to vocalize the hate that lurks within. But, in a country that enshrines free speech in the first amendment, holding and articulating vile views should never be a crime.

After Oklahoma City, the FBI made monitoring domestic terrorism a priority. That emphasis died as the Twin Towers toppled and the bureau was transformed into a single-minded bulwark against al-Qaida and its successors. Even after this sad-eyed weekend, it is hard to see the FBI effectively monitoring rightwing domestic terrorism as long as Trump is president and someone like Jeff Sessions presides over the justice department.

In an ideal world, Republicans of good will (and in a democracy it is imperative to believe they still exist) would bravely and consistently repudiate the bile that fills the swamps of Trumpism and Fox News. For as domestic terrorism stalks synagogues, black churches and the mail of Trump opponents, it is imperative to confront the enduring truth of the dictum – popularized by the comic-strip character Pogo – “We have met the enemy and he is us.”