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

“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.