In an explosive turn of events, President Joe Biden announced Sunday that he is no longer running for re-election, leaving Vice President Kamala Harris as the Democratic Party’s most likely contender to face off against Donald Trump.

“My fellow Democrats, I have decided not to accept the nomination and to focus all my energies on my duties as President for the remainder of my term,” Biden wrote in a statement before endorsing Harris to take over at the top of the Democratic ticket. “My very first decision as the party nominee in 2020 was to pick Kamala Harris as my Vice President. 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.”

Biden’s exit from the 2024 campaign ends a nearly monthlong saga that began with his disastrous debate performance, in which the president appeared dazed, stumbling and stammering through his answers, and got steamrolled by Trump’s usual blitzkrieg of lies. Longstanding concerns about Biden’s mental fitness and electability reached a fever pitch, pushing Harris into the spotlight as the heir apparent.

While Biden publicly threw his weight behind his vice president, Harris will still need to secure the nomination at the Democratic National Convention in late August. She is already on the ticket, so she would seamlessly take over the Biden campaign war chest, avoiding the complicated process of transferring those funds to another candidate. She is also a historic figure, being the first female, first Black and first South Asian American elected Vice President. And Democratic heavyweights, including major fundraisers and top Biden allies like former House speaker Nancy Pelosi and Rep. Jim Clyburn, are expected to quickly rally behind Harris, leaving slim chances for an open convention with a challenger for the nomination.

Still, if there is any open primary process, the Democrats have a deep bench of prospective candidates, including Governors Gretchen Whitmer, Andy Beshear, Gavin Newsom, JD Pritzker, Wes Moore, Josh Shapiro, and Roy Cooper, as well as senators such as Mark Kelly. Any of the aforementioned would presumably be in the running for VP if Harris does ascend to the top of the ticket.

The Biden exit provides Democrats a clean break from concerns about his mental acuity, but Harris brings her own set of challenges. The vice president does perform better against Trump in some polling, but she also suffers from low approval ratings, according to the latest FiveThirtyEight averages.

Her tenure as Vice President has been dogged by mixed reviews, with the criticism largely centered around Harris’ alleged absence from the public stage, high turnover among her staff, and lack of results on key issues in her portfolio like immigration.

Throughout the campaign, however, Harris has emerged as a forceful voice on reproductive rights, a galvanizing issue for Democratic voters since the conservative-majority Supreme Court overturned Roe v. Wade in 2022; and she has taken charge of outreach to blocs that have begun to waver on Biden, including Black, Latino, and young voters. She has also recently become a fixture among the hyper-online, thanks to progressive content creators turning her more awkward moments into fun and relatable memes.

Harris’ rise to the top of the Democratic Party has been three decades in the making. She started as a prosecutor in the Bay Area, working her way up to serving two terms as San Francisco’s district attorney. She then became California’s attorney general, positioning herself as a reformer while often taking tough-on-crime stances, some of which came back to haunt her unsuccessful 2020 Democratic primary campaign.

Yet Harris’ success on issues like crime reduction and consumer protection propelled her to the U.S. Senate, where she earned a reputation for incisive grillings of key Trump-era witnesses, including the former president’s Supreme Court pick Brett Kavanaugh and his attorney general Bill Barr.

And even though her 2020 presidential bid tanked before the primaries even began, Biden selected Harris as his running mate thanks to her formidable debate skills, her governing record, and her broad appeal as both an South Asian American and Black woman. Four years later, Harris may be the Democrats’ best hope to beat back Trump and his party’s right-wing vision for America.