Democrats Find Trump to Be a Uniter, of Their Party

The cross-ideological unity was a dramatic shift from the intraparty fights during the presidential primary that often centered on progressive litmus tests. Mr. Sanders and Mr. Biden may now speak about each other warmly, but during the primary campaign they most clearly embodied opposing ends of the Democratic spectrum on health care policy, breaking up big technology companies, raising taxes on corporations and supporting the expansive climate legislation known as the Green New Deal.

In some ways, the message of the first night’s lineup was more indicative of the 2018 midterm elections, during which moderate victories existed alongside progressive gains, and the Democratic Party swept to power with a big-tent message of holding Mr. Trump’s administration accountable.

Mondaire Jones, a lawyer who won the Democratic primary in New York’s 17th Congressional District and is most likely headed to Capitol Hill in the fall, said both can be true: There is reason for progressives to feel good about Mr. Biden and to be ready to push his administration.

“There is cause for optimism,” he said, adding that there were also areas where Mr. Biden and his running mate, Senator Kamala Harris of California, “are lagging behind American public opinion, in areas like ‘Medicare for all.’ And both of them will have the most progressive Congress in the history of the United States to pressure them and hold them accountable.”

Matt Bennett, co-founder of the center-left political group Third Way, said it was vital for Democrats to give Mr. Biden space to make overt appeals to conservative and moderate voters.

“Anytime you’re expanding the size of the tent, that’s helpful,” Mr. Bennett said. “There are not a lot of persuadable voters late, but to the extent there are any, you have to give them permission to do what they know is right.”

The inclusion of several conservative voices was the most explicit attempt at this persuasion. In a section of the night called “We the People Putting Country Over Party,” the largest set of speakers of the evening, several Republicans and Democratic moderates planned to make a case for Mr. Biden in terms more personal than political.