He’s the oldest president in US history and his approval ratings have stalled in the low 40s. Is President Joe Biden ripe for a primary challenge in 2024?

Donald Trump, the man he defeated two years ago, has just announced he will run for the White House again.

But both men are unpopular, and polls show a majority of Democrats want their party to nominate someone other than Mr Biden.

That may present an opportunity to a generation of young liberals.

Kamala Harris

Perhaps nobody would benefit more from an aging president stepping aside than his deputy Kamala Harris.

A former prosecutor who went on to serve as California’s attorney general and as its junior US senator, the 58-year-old broke glass ceilings in 2020 as the first female, first black and first Asian-American vice-president in US history.

Last year, she briefly served as acting president in the 85 minutes it took Mr Biden to undergo a colonoscopy – but it may be the closest she gets to the Oval Office.

Ms Harris is even less popular than the president and some voters have questioned her competence.

Since taking office, she has been assigned some of the administration’s toughest portfolios, including the influx of immigrants at the southern US border. Conservative critics have accused her of awkward public appearances. High staff turnover and reports of low morale in the VP’s office have only made matters worse.

But the vice-president’s supporters – including an army of online fans sometimes called the “K-Hive” – insist she has been unfairly maligned with misogynistic and sexist slander.

Gavin Newsom

If VP Harris was once a shoo-in to succeed President Biden, it is another California politician – Governor Gavin Newsom – who is now garnering the most 2024 buzz.

Mr Newsom, 55, first gained national attention when, as mayor of San Francisco in 2004, he issued same-sex marriage licences in violation of state law.

A wine entrepreneur by trade, he was elected governor of his home state in 2018 and established himself as a progressive bulwark against the conservative Trump administration.

He earned praise for assertive leadership at the beginning of the pandemic, but a caught-on-camera dinner in which Mr Newsom flouted his own Covid rules gave rise to an effort last year to recall him as governor before his term ended.

Ultimately defeating the Republican-led recall, the governor is now entering a second term after he won re-election earlier this month.

Mr Newsom has a massive campaign war chest and the backing of major Democratic donors, but it is his willingness to pick fights on the national stage recently – from launching ads in Republican-held states like Florida and Texas to criticising his own party for its weak messaging – that is drawing attention to his future ambitions.

Pete Buttigieg

Of the 28 Democrats Mr Biden defeated to become the party’s nominee for president in 2020, few shone as brightly as Pete Buttigieg.

Despite being unknown in the national politics arena, the 40-year-old out-performed seasoned politicians before dropping out of the race and endorsing the eventual president.

Voters were drawn to his resume, youth and polished speech. A Harvard graduate who attended Oxford on a Rhodes Scholarship and served with the US Navy in Afghanistan, Mr Buttigieg had come out as gay in 2015 while serving as the mayor of South Bend, a small industrial town of about 100,000 people in Indiana.

Dubbed “Mayor Pete” by his supporters, he was chosen as Mr Biden’s Secretary of Transportation.

While the cabinet position is typically low-key, Mr Buttigieg has overseen the passage of major infrastructure investments, been forced to respond to various supply chain disruptions and is one of the administration’s most frequent surrogates on cable news.

In the meantime, he and his social media-savvy husband Chasten have also welcomed twins and are often seen hobnobbing in Washington.

Bernie Sanders

If Mr Biden’s age is a deal-breaker for voters, nominating Bernie Sanders – the 81-year-old senator from Vermont – may not make sense to many.

But many voters still see the long-tenured lawmaker as the beacon of a burgeoning left-wing movement in the United States.

Mr Sanders – an independent lawmaker who votes with Democrats – nearly beat the odds in the 2016 presidential nominating contest against Democratic heavyweight Hillary Clinton.

Placing second again in 2020 behind Mr Biden, the self-described “democratic socialist” has since worked to influence administration policy, with moderate success.

But his message that Democrats do too little to convince young voters and the working-class to vote for them continues to resonate, and supporters of another White House run may be hoping the third time is the charm.

Others who could run

Alexandria Ocasio-Cortez: Known to her legions of adoring fans as “AOC”, the youngest woman ever to serve in Congress will turn 35 – the minimum age to be US president – one month before the 2024 election. A darling of the progressive left, Ms Ocasio-Cortez has big ideas and plenty of supporters, but she has demurred on questions about her future, recently telling GQ that her time as a lawmaker has given her “a front-row seat” to the misogyny of American society.

Ro Khanna: A self-described “progressive capitalist”, this Indian-American lawmaker represents the heart of Silicon Valley in the halls of Congress. He has indicated he will not run if Mr Biden is the nominee.

Amy Klobuchar: The senator from Minnesota, 62, did not make waves in the 2020 presidential race, but she is closely aligned with the Biden administration and is a key Democratic power-broker in the Senate.

Cory Booker: Another candidate in the 2020 presidential race, the black New Jersey senator, 53, drew praise earlier this year for an emotional speech, in which he wiped away tears as he celebrated the historic nomination of the Supreme Court’s first black female justice.

Elizabeth Warren: An early pace-setter in the 2020 race, the 73-year-old senator from Massachusetts has channelled female voters’ anger in recent months over the overturning of abortion rights in the US.

Gretchen Whitmer: This Michigan native, 51, faced a kidnapping plot in her first term as the state’s governor, then breezed to re-election earlier this month. She says she has no plans to ever run for president.

JB Pritzker: The Illinois politician, 57, whose family owns the Hyatt hotel chain, is the wealthiest governor in the country. He has targeted Mr Trump in recent speeches, accusing him of “treasonous insurrection”.

Jared Polis: The first US governor to be in a same-sex marriage, Mr Polis, 47, from Colorado, won plaudits for an even-handed approach to the pandemic and has just handily won re-election.

Phil Murphy: A Goldman Sachs financier and former US ambassador to Germany, the New Jersey governor, 65, has worked closely with both Republican and Democratic administrations in Washington.

Every election cycle, pundits opine that former First Lady Michelle Obama or former Secretary of State Hillary Clinton, a twice-failed presidential candidate, could join the race. But while both women remain popular with Democratic voters, neither have so far indulged in stoking the speculation. In fact, Mrs Obama recently told BBC News it was her least favourite question.