Why did working-class voters choose a selfish, thin-skinned, petulant, lying, narcissistic, boastful megalomaniac for president?
One explanation focuses on economic hardship. The working class fell for Mr. Trump's economic populism.
A competing explanation -- which got a boost this week from a study published by the National Academy of Sciences -- dismisses economic hardship and blames it on whites' fear of losing status to blacks and immigrants. They were attracted to Mr. Trump's form of identity politics: bigotry.
If Democrats accept the bigotry explanation, they may be more inclined to foster their own identity politics of women, blacks and Latinos. And they'll be less inclined to come up with credible solutions to widening inequality and growing economic insecurity.
Yet the truth isn't found in one explanation or the other. It's in the interplay between the two.
Certainly many white working-class men and women were -- and still are -- receptive to Mr. Trump's bigotry. But what made them receptive? Racism and xenophobia aren't exactly new to American life. Fears of blacks and immigrants have been with us since the founding of the republic.
What changed was the economy. Since the 1980s, the wages and economic prospects of the typical American worker have stagnated. Two-thirds now live paycheck to paycheck, and those paychecks have grown less secure.
Good-paying jobs have disappeared from vast stretches of the land. Despite the low official unemployment rate, there are millions who are either too discouraged to look for work or work part-time but want full-time jobs.
When I was secretary of labor in the 1990s, I frequently visited the Rust Belt, Midwest and South, where blue-collar workers told me they were working harder than ever but getting nowhere.
Meanwhile, all the economy's gains have gone to the richest 10 percent, mostly the top 1 percent. Wealthy individuals and big corporations have, in turn, invested some of those gains into politics.
As a result, big money now calls the shots in Washington -- obtaining subsidies, tax breaks, tax loopholes (even Trump promised to close the "carried interest" loophole, yet it remains) and bailouts.
The near-meltdown of Wall Street in 2008 precipitated a recession that cost millions their jobs, homes and savings. But the Street got bailed out and not a single Wall Street executive went to jail.
The experience traumatized America. In the two years leading up to the 2016 election, I revisited many of the places I had visited when I was labor secretary. People still complained of getting nowhere, but now they also told me the system was "rigged" against them.
A surprising number said they planned to vote for Bernie Sanders or Donald Trump -- the two anti-establishment candidates who promised to "shake up" Washington.
This whole story might have been different had Democrats done more to remedy wage stagnation and widening inequality when they had the chance.
Instead, Bill Clinton was a pro-growth "New Democrat" who opened trade with China, deregulated Wall Street and balanced the budget. (I still have some painful scars from that time.)
Barack Obama bailed out the banks but not homeowners. Obamacare, while important to the poor, didn't alleviate the financial stresses on the working class, particularly in states that refused to expand Medicaid.
In the 2016 election, Hillary Clinton offered a plethora of small-bore policy proposals -- all sensible but none big enough to make a difference.
Into this expanding void came Mr. Trump's racism and xenophobia -- focusing the cumulative economic rage on scapegoats that had nothing to do with its causes. It was hardly the first time in history a demagogue has used this playbook.
If America doesn't respond to the calamity that's befallen the working class, we'll have Mr. Trumps as far as the eye can see.
A few Democrats are getting the message, pushing ambitious ideas like government-guaranteed full employment, single-payer health care, industry-wide collective bargaining and a universal basic income.
But none has yet offered a way to finance these things, such as a progressive tax on wealth.
Nor have they offered a credible way to get big money out of politics. Even if the Citizens United decision isn't overturned, big money's influence could be limited with generous public financing of elections, full disclosure of the source of all campaign contributions, and a clampdown on the revolving door between business and government.
Mr. Trump isn't the cause of what's happened to America. He's the consequence -- the product of years of stagnant wages and big money's corruption of our democracy.
If they really want to stop Mr. Trump and prevent future Trumps, Democrats will need to address these causes of Mr. Trump's rise.
Robert Reich, a former U.S. Secretary of Labor, is professor of public policy at the University of California at Berkeley. Robert Reich's latest book is "The Common Good.” His documentary, "Saving Capitalism," is available on Netflix.