The US president ended his reelection campaign on Sunday after fellow Democrats lost faith in his mental acuity and ability to beat Donald Trump, leaving the presidential race in uncharted territory.

The 81-year-old had been under mounting pressure to stand down since his halting performance in June’s presidential debate against Donald Trump, which heightened concerns over Biden’s ability to win the election.

A succession of verbal gaffes coupled with Biden’s increasingly visible physical frailty served only to intensify doubts among senior Democratic strategists, especially with Trump surging in the polls after the assassination attempt in Pennsylvania.

Biden’s decision to abandon his bid for reelection will open the way for a new generation of Democrats, including 59-year-old Vice President Kamala Harris, to enter the fray.

An Unprecedented Decision

Biden’s decision to exit the race less than a month before his party’s convention and a few months before voters head to the polls is unprecedented in modern political history. The last sitting president to abandon a re-election bid was Lyndon Johnson in March 1968, eight months before the election.

“We’re in uncharted waters,” said Barbara Perry, a professor at the University of Virginia’s Miller Center. “No president has dropped out or died this close to the convention.”

Replacing Biden on the Democratic ticket will likely cause internal tremors as officials maneuver to become his successor. Factions have formed around Harris and prominent governors like Michigan’s Gretchen Whitmer and California’s Gavin Newsom.

Harris, the first female vice president and a woman of color, enjoys strong support among African Americans. However, her approval rating was only 32% in a recent NBC News poll.