As of mid‑February 2024, the major declared presidential candidates included Joe Biden, Donald Trump, Robert F. Kennedy Jr., Cornel West, Jill Stein, Dean Phillips and others—but not Kamala Harris as a presidential candidate. Biden was running for reelection with Harris as his vice‑presidential running mate, not as a separate presidential contender. (en.wikipedia.org)

On July 21, 2024, Biden announced he would not seek reelection and endorsed Harris to replace him at the top of the Democratic ticket. This announcement triggered the launch of the Kamala Harris 2024 presidential campaign and a rapid consolidation of party support around her. (reuters.com) Harris’s campaign is widely dated from this July 21 entry into the race and is noted for being only 107 days long, the shortest general‑election presidential campaign in modern history. (en.wikipedia.org)

Harris quickly became the Democratic presidential nominee, securing the nomination via a delegate roll call in early August 2024 and heading the party’s national ticket against Donald Trump. (en.wikipedia.org) In the November 5, 2024 general election, she won 48.3% of the popular vote and 226 electoral votes, losing narrowly to Trump (49.8% and 312 electoral votes). That level of vote share, Electoral College support, and national prominence clearly qualifies her as a serious contender in the 2024 election. (en.wikipedia.org)

Because Harris was not a presidential candidate as of mid‑February 2024, but later entered the race and became the Democratic nominee and a major general‑election contender, her candidacy is a direct counterexample to Sacks’s prediction that no such new major presidential candidate would successfully enter and become a serious contender in 2024.