WASHINGTON – The first sign of trouble came before the debate even started.

“Folks, how are you?” President Joe Biden said as he shuffled onto the stage. His voice was raspy and thin, his movements stiff. “Good to be here. Thank you.”

It was an inauspicious glimpse of what would become a historic night. The first and last debate between Biden and Donald Trump started a chain reaction leading to Vice President Kamala Harris replacing Biden at the top of the Democratic ticket. She’ll have her own chance in the spotlight Tuesday as she meets Trump in another debate. She’s been hunkered down to prepare with her team, intent on maintaining the momentum that’s breathed new life into Democrats’ chances this year.

Trump, in a recent interview with a New Hampshire radio host, said he was prepared to face Harris after his encounter with Biden.

“He was not good,” Trump said. “I hope she’s not going to be good, too.”

A look back at the June 27 debate holds lessons for Harris and warnings for Trump, whose rambling and occasional incoherence was overshadowed by Biden’s stumbles.

The first hurdle is understanding the optics of the event.

Optics are important in debates. Ever since Richard Nixon lost the first televised presidential debate to John F. Kennedy in 1960, political experts have treated them like visual spectacles. Biden looked pale onstage and he appeared dazed, his mouth slightly agape, when Trump was talking.

“This is an Instagram, Facebook, TikTok society,” said Michael LaRosa, a former spokesperson for first lady Jill Biden. “Visuals matter.”

LaRosa said Harris needs to be poised at all times because images of her reacting to Trump’s comments can be as potent as when it’s her turn to talk. Her presentation will be closely scrutinized because she’s less well-known than Trump’s, meaning that Americans are still making up their minds about her.

Biden also struggled to make a lucid case for his platform or land solid attacks on Trump.