I would put the starting point even earlier, and note that the problem is not just bad luck but poor leadership, which began in the late 1990s and was compounded by events. Here are the 10 most important setbacks:
1998-9: Clinton impeachment Our present-day political rift really began with the impeachment crisis. It is almost customary to blame the Republican Party–and, ultimately, the electorate seemed to be less interested in President Bill Clinton’s lies under oath than the GOP leadership was. Yet Clinton could have saved the country the trouble by resigning. In clinging to power, he let the country be split and destroyed his own political legacy.
1999: Seattle WTO protests The contemporary radical left, which brought us the anti-war movement, the Obama campaign, and eventually Occupy Wall Street, was born in the streets of Seattle, when labor unions, environmentalists and anarchists united to shut down trade talks that could have boosted global prosperity. To their shame, President Clinton and other leaders let the thugs outside shut down the deliberations within.
2000: Florida recount Though George W. Bush deserved to win, given the actual votes cast, the process through which the decisive electoral votes were granted only served to sharpen bitterness at the fact that he won despite losing the popular vote. Opponents called Bush’s win an “electoral coup” and treated him as illegitimate, insisting he had no mandate. Subsequent attempts by Bush to govern in a moderate fashion were ignored.