It entirely depends on who you would vote for if pushed to vote for the viable candidates. If you would vote for Harris but don’t vote, it helps Trump because that’s one less vote he needs to beat. If you would vote for Trump but don’t vote, it helps Harris because that’s one less vote she needs to beat. So it’s true for every individual’s worst candidate.

When campaigns/people use this message, they’re usually pretty confident them and the person they’re talking to agree on who is the worst.