I think he spoke to people who always felt that way, and then poured more fuel on the fire with his tone and with the economic tension that was already going on anyway.
But he didn't start the fire, it was already there and had been going for a while.
This is my take, for what it's worth, I'm neither very knowledgeable or interested in the intricacies of American politics, so mine is a layman's take.
As you rightly (imo) say it didn't start with Trump, while Trump pedals his conspiracies of election fraud, remember Hillary Clinton told Democrats there was "a vast right wing conspiracy" trying to destroy her husband's presidency, this is Bill Clinton who as president disgraced the White House, perjured himself and became only the second president to be impeached, then there's Bush who pushed the idea of Saddam Hussein possessing weapons of mass destruction as a pretext for the invasion of Iraq.
American political leaders have been playing loose with the truth, deepening partisan divisions and whipping up anger amongst their supporters for decades.
Trumps dangerous delusions and his crazed followers should remind us that America has always teetered on the edge of collapse, a nation born in crisis and awash with bloodshed. Let's not forget it was created by revolution, torn apart by civil war and has seen presidents assassinated.
The 1960s were marked by violence, revolt and political killings and they lit the fuse for division and tribalism. Americans formed battle lines, shouting each other down over black civil rights, gay equality, family values, gun laws, abortion or feminism. I think the election of Nixon revealed a deep division among the American people and profoundly reshaped American politics. The Democrats lost the white working class that was captured by an increasingly conservative and religious Republican right.
Today it's a country whose electoral college system can ignore popular will, whose Supreme Court is politically stacked, where power is held and passed around like a family heirloom. The constitution needs an overhaul. Congress doesn't work, the House is not truly representative and the Senate has too much power.
To the outsider it appears America is locked in a perpetual culture war, lacerated by class, race and faith. The Union survives by compromise.
Something that Callie posted earlier got me thinking, America appears as a nation that refuses to be governed so is it time to break up the Union?. To allow The United States to disintegrate, fracture and break apart?.