If Joe Biden drops out, who could replace him?

If - and it's still a big if - beleaguered President Joe Biden drops out of the US presidential race just over 100 days before the 5 November election, who might replace him?

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US Vice President Kamala Harris

Harris is the obvious front-runner for many reasons - many vice presidents have gone on to hold the top office, and being the first female VP, the first VP of Black and Asian descent, dumping her from the ticket could backfire horribly. At 59 years old, she's young - but not as young as Donald Trump's new running mate JD Vance, who is only 39. Despite some mixed reviews about her performance over the last three-and-a-half years, she's proven to be a fiery and energetic campaigner on the trail and even with all the intrigue going on behind the scenes, she's been publicly loyal to Biden. Also, here's an important note - the millions of dollars in the Biden-Harris campaign fund could be easily transferred to her if she stays on the ticket, as her name is on all the registration documents. With another candidate, that could be trickier.

On the other hand, Biden might not choose to officially anoint Harris as his successor, and the Democrats could open up their convention in late August to all comers. That could be chaotic, or it could be a valuable burst of energy. If the race opens up to all, here are the most likely names we'll be seeing.

California Governor Gavin Newsom

Perfect hair, perfect teeth, and the governor of America's largest state - Newsom, 56, clearly has presidential ambitions. He even met with Prime Minister Christopher Luxon just last week. However, there are concerns a flashy West Coast liberal doesn't have a chance in the current deeply divided US, and the American conservative media will make hay out of the fact his ex-wife Kimberly Guilfoyle is now a passionate Trump fan who is engaged to Donald Trump Jr. He may well run in 2028, but Harris also being from California means that due to complicated US residency rules it's very unlikely Harris and Newsom could run together easily this time out.

Michigan Governor Gretchen Whitmer

Whitmer is perhaps the most likely candidate to run if Harris stumbles or bows out. The Michigan governor is relaxed, popular and has the nickname "Big Gretch". She's just released her memoirs, which is practically a requirement before running for president. She was the target of a plot to allegedly kidnap her by a militia, so she's used to tough battles. And at 52, she would definitely represent a passing of the torch to a younger generation.

Pennsylvania Governor Josh Shapiro

Shapiro, 51, narrowly won election in 2022 against a Trump-backed Republican, and has received plaudits for working across party lines and actually achieving results in office. He also delivered a strong response after last weekend's assassination attempt against Donald Trump in his state. But he's also been governor barely two years.

Former US First Lady Michelle Obama

This is a West Wing-style fantasy by many out there on the internet, and almost entirely unlikely to ever happen. The highly popular former US First Lady, 60, has never expressed a desire to hold elected office, and has denied any interest. She told Oprah Winfrey last year "politics is hard" and told the BBC in 2022 she detests being asked if she'll run for president.

Other dark horses mentioned out there include - for the eternal optimists - past candidates like Hillary Clinton and Senator Bernie Sanders, plus rising stars like Transportation Secretary Pete Buttigieg, Maryland Governor Wes Moore, Illinois Governor JB Pritzger, Kentucky Governor Andy Beshear and North Carolina Governor Roy Cooper - the latter two both also being mentioned a lot as potential Harris running mates.

But while the internet is filled with gigabytes of speculation, right now the ball is firmly in the court of sitting President Joe Biden - and whatever call he makes is likely to be the biggest tremor yet in the tectonic shifts in the US presidential campaign seen in the past four weeks.