In his speech, Joe Biden will preview a fall campaign strategy against Trump.

As the newly minted leader of the Democratic Party, Joseph R. Biden Jr. plans to ride out of its national convention with a message for the fall campaign centered on compassion, linking his heart-rending biography of setback and recovery to the lives of Americans hoping for their own rebound in a season of hardships.

But looming over Mr. Biden’s long-sought presidential nomination on Thursday is the ever-present shadow of another man who’s poised to dominate the final 10 weeks of the campaign and use his considerable megaphone to drown out Mr. Biden’s pitch: Donald J. Trump.

The president, who spent the day attacking Mr. Biden in the swing state of Pennsylvania, has been the greatest unifying force at the Democratic convention, given that loathing him is the one thing that everyone in the fractious party can agree on.

Mr. Biden and the Democrats spent the week prosecuting arguments against him on Covid-19, unemployment, health care, child care, climate change, foreign policy and his fundamental fitness for the presidency — attacks that only presaged a fall campaign that, even when it features Mr. Biden, will be aimed entirely at drawing contrasts with the current occupant of the Oval Office, Democrats say.