A perfect storm in the Democratic Party is making a once-unfathomable scenario plausible: a political comeback for Hillary Clinton in 2024. Several circumstances—President Biden’s low approval rating, doubts over his capacity to run for re-election at 82, Vice President Kamala Harris’s unpopularity, and the absence of another strong Democrat to lead the ticket in 2024—have created a leadership vacuum in the party, which Mrs. Clinton viably could fill.…