The V.P. choice was both conventional and groundbreaking.

By choosing Ms. Harris as his running mate, Mr. Biden opted for a time-honored model in which running mates are not just governing partners but political understudies of sorts.

Pegged as a rising star for a decade, but with less than four years of experience in the Senate — she was 8 years old when Mr. Biden was first elected to the chamber — Ms. Harris, 55, reflects a traditional archetype in an election year that has been anything but normal.

She is also a thoroughly establishment-friendly figure, as is Mr. Biden: Both have hewed closely to their party’s mainstream for years, shifting left with the times but always with an eye on the broader electorate and higher office.

Progressive Democrats now find themselves led by two moderates with relatively cautious political instincts, even as activist energy courses through the party and left-wing challengers unseat some incumbents. The party establishment is hoping that the mostly young protesters filling the streets of nearly every American city to denounce police brutality and Mr. Trump will rally behind two figures who have offered sympathetic words and proposals but whose careers have been shaped by their relationship with law enforcement.

“She’s not of the far left of the party, she’s a former prosecutor,” Janet Napolitano, the former Arizona governor and Homeland Security secretary, said of Ms. Harris. “And when you’re a prosecutor, you have to make some tough calls.”

Mr. Biden also chose Ms. Harris to help inject excitement into his campaign. He is leading in the polls, but mostly because he’s the genial alternative to the most divisive president in modern history, who is presiding over a pandemic and economic collapse.

Mr. Biden also turned to Ms. Harris to bring a fresh perspective to the West Wing should they win — a similar calculation, but with the roles reversed between ticketmates, that propelled him to the vice presidency 12 years ago.

Mr. Biden spurned those progressives who wanted their consensus-oriented standard-bearer to elevate a liberal like Senator Elizabeth Warren, instead picking a prominent leader from the demographic that resurrected his campaign in the Democratic primary. By doing what Hillary Clinton did not do four years ago and choosing a Black running mate, he may give the party’s most loyal voters a reason, beyond animus toward Mr. Trump, to work for and elect the ticket.