Why are third party candidates a threat to Biden in 2024?

This week, Joe Biden admitted that he probably would not be running for re-election if Donald Trump was not likely to be the Republican candidate. The thoughts of a rehashed presidential race in 2024 has many Americans dreading next year, and some are looking to third-party or independent candidates as potential alternatives. So why hasn’t an outsider been more successful in the past? Is running independently of the Democrat and Republican parties a legitimate offer to voters, or nothing more than an election spoiler? And if the answer is the latter, why should the president be the one to worry? This week, Jonathan Freedland speaks to Nitish Pahwa of Slate about why Democrats are worried that Biden could suffer the same fate as Hilary Clinton in 2016