When the US Postal Service delivered over a dozen potentially lethal pipe bombs to public figures last week, Americans were immediately reminded that all of these intended victims, from former President Barack Obama to former CIA director John Brennan, have been frequent targets of the demagogic rhetoric of Donald Trump.

As it emerged that the bomber was a 56-year-old Floridian with shaky mental health, a substantial criminal record and a van completely covered with stickers proclaiming his allegiance to Trump, the president characteristically denied any responsibility. “There’s no blame. There’s no anything,” Trump said blithely, dismissing as usual any connection between his rabble-rousing rallies and the responses of his supporters.

Trump made a feeble and half-hearted attempt at acting presidential on Friday, urging civility and national unity. That did not last long. He soon reverted to inveighing against many of the same people and institutions the bomber had targeted. Trump even advanced a theory that liberal elements had staged the event to scare voters in advance of the imminent US mid-term elections.

This president is historically divisive and inflammatory. Handed two moments when he could have spoken reassuringly to the country, he turned the pipe bombs into a political farce and reflexively dismissed any suggestion that the tragic shooting over the weekend at a Pittsburgh synagogue compelled more responsible gun control legislation.

It can be fairly debated that many of Trump’s policies and initiatives actually need to be considered. But this president’s personal conduct and public commentary demean his office and the country he was elected to lead and serve.

How did the United States get to a point where such a person could even contemplate running for elective office, securing the nomination for president from a major political party and, astonishingly, winning the election? What happened to the America that elected George HW Bush and Ronald Reagan?

These questions have of course been on many minds since Trump really emerged as a significant factor on the national political scene roughly two-and-a-half years ago. A rough consensus has begun to emerge to explain this seemingly inexplicable phenomenon.

Let us set aside for a moment the discouraging correlation between Trump’s racist remarks and alleged business history of discriminating against blacks and other minorities, and the Republican party’s cynical 50-year embrace of the racism and bigotry of parts of the American south, rural midwest and western intermountain regions.

To find the roots of the current dysfunction and internecine political warfare in Washington, many observers are recalling the presidency of George HW Bush, who assumed office in January 1989. Nominated and elected largely on the recommendation of his popular predecessor Reagan, Bush had no sooner entered the White House, it seemed, when the Soviet Union collapsed, doomed by its own bankrupt ideology and policies. The Berlin Wall fell. The West had won the Cold War. Bush then presided over the last big US military victory, in the first Gulf War, so America reigned triumphant.

Bush looked invincible heading into the 1992 presidential election. Perhaps this gave him the courage to cut a remarkable deal with Congress to balance the American budget, heralding a decade of US prosperity. But the seeds of change had been sown. Three political newcomers arrived to help set the political stage for the rise of Donald Trump over 20 years later.

Leading Democrats refused to challenge Bush in 1992. This allowed unknown Governor Bill Clinton of Arkansas to seize his party’s nomination. Around this same time, another unknown politician, Georgia congressman Newt Gingrich, led a back bench revolt of Republican lawmakers to challenge the political establishment and break apart the last era of bipartisanship. Clinton’s personal scandals and Gingrich’s anarchic ambitions gradually shattered prior political norms.

And finally, H Ross Perot, an eccentric Texas billionaire, ran in 1992 and 1996 as the candidate of the so-called Reform Party. His platform was stridently anti-establishment, opposed NAFTA and mocked the political party duopoly that had prevailed for so long in the US. Perot received many millions of votes in those two elections.

A figure from New York’s tabloid newspapers considered running for the Reform Party in 2000. But Donald Trump decided the time was not yet right. He would pick his time when he felt it was right - and here we are.