President Biden and Vice President Kamala Harris showcased the passing of the Democratic torch at their first joint public appearance since he left the race, celebrating a victory on lowering drug costs at an event on Thursday where the two leaders projected an enduring partnership even as the spotlight settled squarely on her.

Ms. Harris spoke first, introducing Mr. Biden as an “extraordinary” human being and appearing to say “I love you” when she turned over the microphone to him. In turn, Mr. Biden made a forceful case for his anointed successor, saying she would make “one hell of a president.”

Mr. Biden, who is seeking to cement his legacy in his last months in office, trumpeted the administration’s Thursday announcement about major drug price negotiations between Medicare and pharmaceutical companies, which will take effect in 2026. The negotiations covered the prices of 10 costly or common medications taken by millions of older Americans, including widely used blood thinners and arthritis medications.

He said the new policy — a result of the Inflation Reduction Act, one of his signature legislative accomplishments — reflected fights that he had taken up for years against pharmaceutical companies.

But Mr. Biden made it clear that part of his legacy included Ms. Harris, whose campaign for the presidency will ride partly on policy achievements she helped deliver during his presidency. Ms. Harris cast the tiebreaking vote that allowed the Inflation Reduction Act to pass.

“Kamala and all of us in this room are going to keep standing up to Big Pharma,” Mr. Biden said. “I fought too damn hard to yield now.”