“Our focus now should be on ensuring that we get the most bold, progressive agenda possible from our Democratic candidate — because that is what Americans want,” Ms. Omar said in a statement.

It was an undeniable political success for Mr. Biden to secure the swift endorsement of his chief primary rival, who dropped out of the Democratic primary just last week. The hope among Mr. Biden’s allies is that the move by Mr. Sanders will be a catalyst for a united Democratic Party to defeat President Trump, a sign of party-wide political alignment against a common enemy.

At the very least, Mr. Biden has seemingly avoided problems faced by Mrs. Clinton in 2016, when she spent weeks being needled by Mr. Sanders even after it was clear she would be the nominee. Though Mr. Sanders did endorse Mrs. Clinton, and she embraced some of his more left-wing priorities as he campaigned for her on the trail, she never had the full-throated backing of the party’s progressive base, and fell flat on Election Day.

This time, Mr. Sanders backed Mr. Biden just five days after leaving the race himself. However, for Mr. Sanders’s endorsement to have maximum political impact, his most energized supporters — mainly young people, progressives and many Democrats in Western states with heavy Latino populations — have to come along, too.

Young people were a key part of President Barack Obama’s winning coalitions, but they did not support Mr. Biden in the primary. Polling has consistently shown that Mr. Biden’s voters skewed older and Mr. Sanders had more support not only among Gen Z voters at the far end of the spectrum, but also among millennials and voters up to 45.

The legions of activists who backed Mr. Sanders will also be important in state and local races, as Democrats seek an all-hands effort to win back the state and local seats that they lost during Mr. Obama’s presidency.