Biden Always Had a Simple Message. He Rode It to the Nomination.

Staring into the camera, Joseph R. Biden Jr. recalled the white supremacist protests in Charlottesville, Va., declared that the soul of the nation was at stake in this election and, invoking America’s founding creed, claimed history’s call by vowing to defeat a president who he said threatens democracy itself.

That’s how Mr. Biden accepted his party’s nomination at the conclusion of the Democratic convention Thursday night.

But it’s also how he announced his candidacy, in a three-and-half minute video in April of last year.

In the intervening months, Mr. Biden was occasionally thrown off balance — by attacks on his son Hunter’s work in Ukraine; questions about his antiquated cultural references and gaffe-prone speaking style; and laments that his politics were as dated as some of his favorite sayings.

But he never wavered from his central message: that President Trump was a danger to American values, while he was a stable, experienced leader who represented his party’s strongest option for the general election. Nor did he succumb to the purity tests on policy that coursed through the Democratic field during the 2020 primary season.

“A lot of smart Democratic strategists overcomplicated things,” said Elisabeth Smith, who was a top aide to Pete Buttigieg’s campaign, noting that most anti-Trump voters are animated chiefly by the burning desire to defeat him. Referring to Mr. Biden, she said, “Because of the way he conducted himself in primary, not kowtowing to every demand from the left, he never had to pivot from a primary to a general election message.”

On Friday, Democrats were celebrating a mostly glitch-free convention, happy with Mr. Biden’s performance, many convinced of his theory of the case. As he enters the fall, his relative consistency — the through line from announcement video to acceptance speech — illustrates why he captured his party’s nomination after initial stumbles, and why he has maintained a steady lead in the polls over Mr. Trump.

Whether that’s sufficient for victory in November will now be tested by the president and his allies, who will take center stage next week at a convention that will be conducted in part from the White House. Mr. Trump has relentlessly attacked Mr. Biden and Democrats in recent months, and he continued the assault on Friday, calling the Democratic convention the “darkest and angriest and gloomiest” in the country’s history.

It was an oddly discordant message for a president who himself regularly delivers exaggerated portraits of carnage in American cities and dark predictions of calamity under Democrats. Even on Friday Mr. Trump quickly pivoted to say that, if he’s not re-elected, “no one will be safe in our country,” and called himself “the only thing standing between the American dream and total anarchy, madness and chaos.”

It was only the latest example of how Mr. Trump has struggled to settle on a single message against Mr. Biden. In contrast to Mr. Biden’s relatively disciplined, 16-month campaign against him, Mr. Trump has at times portrayed his rival as an overly close ally to China, a political lifer beholden to Washington’s status quo, a radical liberal and a doddering old man.

Since the president’s installation of a new, more experienced campaign manager, Bill Stepien, his scripted remarks and advertising have steadied. Mr. Trump has caricatured Mr. Biden as being a tool of his party’s extremists, if not one himself, and highlighted the spasm of rioting in some cities to try to gain back some wavering Republicans in the polls.

When the election cycle began it wasn’t clear Mr. Biden’s strategy would work. At times the consensus-minded Mr. Biden seemed out of sync with a party that was steadily moving left. He was naïve to believe that voters only wanted a return to normalcy and was overly fixated on Mr. Trump — at least that’s what his legion of intraparty critics said.

But by the time the convention began this week, Senator Bernie Sanders, the democratic socialist, had rallied to him because he shared Mr. Biden’s alarm over Mr. Trump. So too did a bevy of Republicans who praised Mr. Biden, denounced Mr. Trump and gave tacit permission for Republican-leaning independents and even some G.O.P. voters to support a Democrat.

From the start, the 77-year-old former vice president and his advisers had a better feel for the Democratic primary electorate than their skeptics, or their rivals.

Mr. Biden himself never put it this way, but the case for his candidacy was clear enough from hundreds of conversations with voters: he’s a decent man who was a loyal lieutenant to President Obama, has suffered some tough knocks along the way and has the best chance to end this turbulent period by defeating Mr. Trump.

“Biden understood how much good will there still was for Obama,” said Ms. Smith, contrasting him with those Democrats who “were hitting Obama from the left.”

In the primary, Mr. Biden declined to join his competitors who called for decriminalizing the border and eliminating private health insurance, not wanting to give away his party’s advantage on two issues — immigration and health care — that lifted them to success in the 2018 midterms.

Since he emerged as his party’s standard-bearer, he’s again made it harder for Republicans to link him to the left, spurning calls to defund the police and tear down statues of America’s founders.

This is not to say Mr. Biden has the election wrapped up.

The uncertainty around voting caused by coronavirus, Mr. Trump’s determination to undermine mail-in ballots and the country’s polarization leave more room for suspense than in past races. So too do the fall debates, where Mr. Trump will get a chance to attack Mr. Biden and perhaps trip him up — and make some people wonder if he is fully up to the presidency.

The convention allowed Mr. Biden to deliver his message to a broad audience. But he enters the final two months of the race committed to avoiding most in-person campaigning, while Mr. Trump and the Republicans are returning to the road. That strategy risks handing the president an even larger megaphone than he already enjoys. It could prove difficult to convey his message about defeating Mr. Trump and confronting the country’s health and economic crises if he’s being drowned out each day by a president who loves center stage.

Message discipline also becomes even more crucial in the final weeks of a general election. Hillary Clinton and her campaign had to deal at times with challenging distractions in the fall 2016, from Mr. Trump’s endless attacks to hacked emails to James Comey, and John McCain was thrown off message when the financial crisis hit in 2008. John Kerry was upended by the Swift Boat controversy and didn’t combat it aggressively enough.

Mr. Biden’s lack of campaign events and regular press conferences has helped him stay on message, but the debates and 10 weeks of news cycles are bound to bring unexpected moments that can’t be answered simply with metaphors about darkness and light.

Should the former vice-president win in November, though, it will be in part because he shared a hallmark of other recent presidents. He knew why he was running, articulated it from the outset, and never abandoned his core message.

Bill Clinton did the same in 1992, presenting himself from the start as a different kind of Democrat. George W. Bush ran as a “compassionate conservative” through the primary and general election eight years later. And for all their differences, Mr. Obama and Mr. Trump did not drastically change their appeals from their first day as presidential candidates.

Importantly, Mr. Biden has also married his message to who his voters were and what motivated them.

He counted on his multi-racial coalition propelling him, and even when his campaign seemed to have run aground his aides pointed to South Carolina, predicting moderate whites and Blacks there would vault him to Super Tuesday.

Some Black voters, in particular, were sobered by Mr. Trump’s victory and convinced that white America would only replace him with a moderate white man.

“It was simply, ‘Oh my God this man is an existential threat and we just to have to focus on who has the best chance of beating him,’” Cornell Belcher, a Democratic pollster, said of Mr. Trump. “And they thought Biden had the best chance of doing that, hard stop.”

On Thursday Mr. Biden quoted from one of his favorite poems, about how “once in a lifetime” we see “hope and history rhyme.” He was talking about his vision for the country but he could have been talking about himself and the confluence of forces that lifted him to the nomination on his third attempt.

Mr. Biden recognized that Mr. Trump was the raison d’être for his candidacy and therefore Mr. Trump should be the centerpiece of his message.

But it almost didn’t start that way. As he prepared to announce his candidacy, Mr. Biden hired a talented Democratic advertising consultant to craft a biographical announcement spot. They recorded footage last year in Scranton, Pa., the former vice president’s childhood hometown.

Mr. Biden didn’t like it, and he made one of his longtime advisers, Mike Donilon, create a new video with ominous images of torch-bearing neo-Nazis in Charlottesville. It set him aloft in the primary, and on to the nomination and perhaps the presidency.

He knew what he wanted to say.