Dietrich, like even the most circumspect observers of the 2020 campaign, does not predict that Biden will fall apart. But Democrats carry checklists in their minds of the universe of things that could alter the course of the campaign.

Biden might say the wrong thing at a debate, or have an awkward moment in an interview or at a press conference. Trump’s massive advertising campaign might begin to resonate, hurting Biden’s favorability ratings. Biden’s campaign might make poor decisions about spending allocations in the battleground states, or the coverage of his campaign may sour if he loses even a percentage point or two in polls. Presidential candidates with large leads have all suffered from less.

And then there are the factors outside of Biden’s control. It is possible that Trump before November will announce a coronavirus vaccine, whether real or imagined. And it is possible that the economy will improve, a prospect Republicans are pinning their hopes to.

So much has changed over such a short period of time — so far, much of it to Biden’s advantage — that it’s impossible to rule out any kind of black swan political event.

Late this week, Les Francis, a Democratic strategist and former deputy White House chief of staff in the Carter administration, sent an email to a circle of friends, including a former congressman and former administration officials, with the subject line, “123 days until the election — and a sobering prospect.”

Right now, he said, “Trump is more than vulnerable.” But then he went on to outline a scenario in which Republicans hold down turnout and sufficiently harden Trump’s base.

“Think it can’t work?” Francis concluded. “Think again.”

Biden’s polling lead over Trump is significant, but not unprecedented. The RealClearPolitics polling average has Biden running ahead of Trump by just less than 9 percentage points.

Richard Nixon maintained double-digit leads over Hubert Humphrey throughout the summer of 1968, then was forced to scramble in the fall as Humphrey surged. Twenty years later, following that year’s Democratic National Convention, a Gallup Poll put Michael Dukakis’ lead over George H.W. Bush at 17 percentage points. As they do today, voters that summer appeared eager for change — before abandoning Dukakis and voting for Bush.

“Sometimes things can look very, very comfortable and it changes, it can change very, very quickly,” said Ken Khachigian, a former aide to Nixon and chief speechwriter for Ronald Reagan. “The psyche of the American voter can be affected by events very dramatically between Labor Day and Election Day.”

If he were running Biden’s campaign, he said, “I’d be feeling pretty good now, but I wouldn’t be buying property in Northwest Washington quite yet.”

Perhaps nothing is more indicative of Biden’s growing advantage than the changing frames of reference required to doubt it. Throughout the Democratic primary, Biden was so widely expected to implode that several other centrist candidates premised their entire campaigns on the expectation. Then came the comparisons to 2016 — and the polls that put Hillary Clinton ahead at a similar point in the campaign. After it became clear that Biden was on stronger footing than Clinton, the unpersuaded reached back further for examples of catastrophe.

Often, they settle on Dukakis and his race against Bush.

In one way, that election is uniquely on point for Biden. It was during the 1987 primary — his first run for president — that a plagiarism scandal engulfed Biden’s campaign, with the discovery he had lifted lines from a speech by British Labour Party leader Neil Kinnock.

“If there’s one thing we learned from ’88, Biden is capable of screwing up big time,” said John J. Pitney Jr., who helped on Bush’s campaign in 1988 and wrote a book about that election last year.

Pitney, who went on to become an acting director of research at the Republican National Committee, said that in the current race between Biden and Trump, “you’d have to rate [Biden] as a decisive favorite at this point.”

However, he said, “What we found in 2016 is even a few points in a few states can make all the difference, so that’s why Biden shouldn’t be counting on napping through September and October.”

So far, Biden appears not to be. He has raised more money than Trump for two months in a row, and his campaign recently went up with its first major advertising offensive of the general election. Biden is taking more steps out of his Delaware home, where he has remained throughout much of the coronavirus pandemic. He said this week that he “can hardly wait” to debate Trump.

Former New Mexico Gov. Bill Richardson, who ran for president in 2008 and was initially skeptical of Biden’s decision to remain cloistered at home, said that there is “no historical context for what’s happening, at least in my lifetime.”

“I thought it was a mistake to run a low-key race,” he said. “But given Trump’s erratic behavior and his miscues … for now, Biden is running a perfect race, which means let Trump be Trump, let him self-destruct.”

Trump has privately acknowledged he's losing, and he is desperate to correct course. Republicans view the debates as an opportunity to gain ground, as Bush did following Dukakis’ emotionless response to a question about the death penalty in the event his wife, Kitty, “were raped and murdered.”

And Trump’s campaign is just beginning to swamp the airwaves with negative ads about the presumptive Democratic nominee. In a campaign not unlike the Lee Atwater-orchestrated assault on Dukakis’ fitness to serve, Trump is airing ads casting Biden as aged and confused, with mental capabilities that are “clearly diminished.”

Phil Angelides, the former California state treasurer who was a major fundraiser for Dukakis and who has bundled money for every Democratic nominee since, said that after Trump’s victory in 2016, “I don’t think we can take anything for granted.”

But Dukakis, he said, was not as well known to voters as Biden. And the economic conditions that year were far better than they are now.

“It was a pretty good environment for the incumbent [party], unlike today,” Angelides said.

If anything, the underlying environment may be historically bad for Trump — so bad he may not only get flattened in November, but he might become the proximate cause of a wholesale shift in the American electorate.

Seniors and suburban voters, two longtime pillars of the Republican coalition, are defecting to Joe Biden. Once-red states suddenly seem competitive, and children of Reagan Democrats are marching in the streets.

“The tectonic plates are shifting, said Chris Lehane, a former Clinton White House staffer who helped to manage the turmoil surrounding that president’s impeachment proceedings. “On June 1, if I had told you that by July 1 the flag would be down in Mississippi, Woodrow Wilson would be off the wall at Princeton, Juneteenth would be a national holiday for companies, Black Lives Matter would reflect the great, not so silent majority, you would question my sanity. That’s all happened in 30 days.”

In the midterm elections, suburban voters revolted against the president. And then came the coronavirus pandemic, which has killed about 130,000 people in the United States. Trump’s favorability rating cratered, and his problems were compounded by the civil unrest following the death of George Floyd. While Trump responded with a stream of “law and order” rhetoric, streets filled with protests amid a national reckoning on race.

“The pandemic’s bad enough for Trump, because he BS’d his way through it,” said Paul Maslin, a top Democratic pollster who worked on the presidential campaigns of Jimmy Carter and Howard Dean. “What George Floyd did is it served to activate this other America to say, ‘Wait a minute, who are we?’”

It is possible that the election will be close, he said. But “it wouldn’t surprise me if it ends up between 8 and 10” points — a landslide for Biden.

Dietrich, at the National Democratic Training Committee, said Friday, “Can we have the election this afternoon? We’d wipe the f------ board with him right now. But polls and momentum, they’re a snapshot … We have absolutely no idea where we’ll be in November.”

Still, he said, “I would rather be us than them.”

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