Vice President Kamala Harris, who has struggled for nearly four years in President Biden’s shadow, was thrust on Sunday into the center of a remarkable political drama that could culminate with her becoming the first Black woman at the top of a major-party presidential ticket.

Mr. Biden’s decision to abandon his re-election bid and endorse Ms. Harris to succeed him puts her in a powerful, but not certain, position to become the new face of the Democratic Party, charged with preventing former President Donald J. Trump from returning to the Oval Office for another four years.

“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,” Mr. Biden wrote in a social media post after he announced his decision to step aside. “Let’s do this.”

Ms. Harris and her team are likely to move swiftly to try to seize that mantle even as uncertainty swirled about whether other Democrats would seek to challenge her for the nomination at the party’s convention in Chicago next month.

In a statement, Ms. Harris thanked Mr. Biden for the endorsement, saying that his “legacy of accomplishment is unmatched in modern American history.” She vowed to “earn and win this nomination” and to keep Mr. Trump from serving another four years in the White House.

“We have 107 days until Election Day,” Ms. Harris wrote. “Together, we will fight. And together, we will win.”