Trump rejected Philadelphia. The city was rewarded.

Two of Mr. Biden’s granddaughters lived in the city and attended the University of Pennsylvania at West Philadelphia, which was also the alma mater for Mr. Biden’s son Beau, who died in 2015, and the daughter. President-elect Ashley, who was reportedly still living in the city last month.

After Mr. Biden’s vice-presidency, the university became a key part of its professional base, establishing the Biden Center for Diplomacy and Global Engagement in Washington.

He was paid hundreds of thousands of dollars as a professor by the university, and several of his close advisers, including potential appointees in the fledgling Biden administration, were repeatedly affiliated with the center. They include Steve Ricchetti, chairman of the Biden campaign and former vice-presidential chief of staff to the president-elect, as well as Brian McKeon, Colin Kahl, Michael Carpenter and Dan Erikson, former advisers to Mr Biden.

When Mr Biden launched his presidential run, he chose Philadelphia for the campaign seat, along with his announcement speech and kickoff fundraiser, which took place at a home in leafy West Mount Airy owned by David L. Cohen, a senior executive at Comcast. Mr. Cohen, who is also chairman of the board of the University of Pennsylvania, participated in the grand opening of the Biden Center.

Mr Biden’s campaign staff included a number of Philadelphia-area natives in prominent roles, including his national political director, Erin Wilson.

To increase turnout in the city, Democrats have pulled all the shutdowns. They called in reinforcements from celebrities like Debra Messing and Kathy Najimy and sent them knocking on doors. They lined up John Legend to sing for a crowded parking lot. And they sent the best political surrogates, including former President Barack Obama and Senator Cory Booker of New Jersey, to campaign in the city.

Local Democrats have increased the effort. Representative Dwight Evans has organized trailers to travel through neighborhoods in his district, which includes parts of North Philadelphia and West Philadelphia, honking their horns to remind residents to vote. On Sunday, his trailer traveled 35 miles. Philadelphia stretches just 15 miles north to south.