From National Review:

American politics has its norms and polite fictions. Presidential candidates are not supposed to admit that they are running for president until they are ready to formally launch, a tradition that has led dozens of candidates to flatly lie about their intentions. Vice presidential prospects are not supposed to openly stump to be considered, in part because it is seen as somewhat beneath the dignity of senators, governors, and recently defeated presidential contenders to admit wanting a second-banana job, partly out of concern for upstaging the nominee.

Stacey Abrams, the unsuccessful 2018 Democratic nominee for governor of Georgia, has clearly decided to ignore that norm and openly campaign for the job.