President Biden is the first sitting president to drop out of running for reelection since Lyndon B. Johnson in 1968 — leaving the race against Donald Trump in turmoil.

“My very first decision as the party nominee in 2020 was to pick Kamala Harris as my Vice President,” Mr. Biden posted on social media. “And it’s been the best decision I’ve made. Today I want to offer my full support and endorsement for Kamala to be the nominee of our party this year. Democrats — it’s time to come together and beat Trump. Let’s do this.”