Monday night's debate could be pivotal in this tight, tense race for the White House. Or not.
To put it crudely: white, college-educated voters are inclined to vote for Hillary Clinton, in coalition with African-Americans, Hispanics and Asians. They are not enthusiastic about the Democratic nominee – same-old, same-old when we need change; couldn't tell the honest truth to save her life – but they are dismayed by Donald Trump's nativist populism.
White voters without a college education are with Mr. Trump. And for many of them, nothing but nothing will change their mind.
Tepid, weak support for Ms. Clinton by a plurality of voters, and fierce commitment to Mr. Trump by a smaller but more motivated base, defines this election. In that environment, Ms. Clinton might help or harm her cause Monday night. Mr. Trump will be Mr. Trump – loathed or adored, but unlikely to surprise.
Think of the things that have damaged Hillary Clinton in the past few weeks. The FBI said she was extremely careless in how she handled classified e-mail on a private server. Her health became an issue when she came down with pneumonia. And she described half of Mr. Trump's supporters as a "basket of deplorables." These embarrassments were sufficient to evaporate her lead in the polls.
Now think of the things that have not damaged Donald Trump. The Washington Post reported that Mr. Trump diverted $100,000 from his charity, the Trump Foundation, to settle a dispute between a private business he owned and the town of Palm Beach, Fla. If this allegation is proven to be true, this appears to be blatantly illegal.
The Associated Press reported that the Trump Foundation donated $25,000 to the re-election campaign of Florida Attorney General Pam Bondi, who solicited the donation. Shortly afterward, Ms. Bondi decided Florida would not join a fraud case against Trump University. Was that donation a bribe? Ms. Bondi hotly denies it.
Last week, after five years of promoting the racist lie that Barack Obama was not born in the United States, Mr. Trump finally accepted that the President's Hawaiian birth certificate was real, and then promptly blamed Hillary Clinton for starting the birther rumours in the first place, an allegation AP called another "false conspiracy."
Yet despite, or perhaps because of, the many lies, insults, baseless accusations and racist policies and taunts, Mr. Trump has the support of more than four Americans in 10.
The simple fact is, so many have lost all confidence in those who investigate and report on his transgressions, especially such once-respected institutions as the Post, AP, the New York Times and any broadcaster other than Fox News, that their reports no longer have the power to influence events, or their opinion writers to change minds.
If so, then what does it matter how Mr. Trump performs in Monday's debate? Trump voters discount evidence of wrongdoing that, in any previous election, would have sunk any candidate, Democrat or Republican. Gaffes and stumbles in a (rigged! they will say) debate won't shake them.
In the populist rebellion Mr. Trump leads, there are no facts, only Us and Them, and anything They say is just the Establishment and its enablers protecting their privilege.
This is exactly what the youthful Left proclaimed 50 years ago. It's what the aging Right proclaims today. History has inverted itself.
There are undecided voters, and Monday's debate could matter to them. They make up between 5 and 20 per cent of the electorate, depending on which pollster you consult.
They include younger voters who would normally vote Democrat but lack any enthusiasm for Ms. Clinton, and older voters who would normally vote Republican but who are wary of Mr. Trump.
Both groups will be assessing Ms. Clinton's vigour and stamina in the debate, and Mr. Trump's depth (or lack) of knowledge on key issues.
They might be moved by what happens. The rest are resolved either to vote with joy for Mr. Trump, or for Ms. Clinton with a sigh.