Anthony Weiner's Sexting Parter -- He Needs To Pull Out

Anthony Weiner is terrible at pulling out ... of mayoral races ... at least according to his sexting partner who firmly believes he should withdraw from the race.

23-year-old Sydney Leathers made headlines this week when it came out that she and Weiner had been sending nudie pics back and forth after leaving Congress. Weiner says he's staying in race.

But Sydney tells TMZ ... Weiner's a cooked goose, saying, "He should pull out." Leathers tells us she believes Weiner is unfit to run the Big Apple.

Leather's has become an advocate, urging New Yorkers to vote for Christine Quinn -- Weiner's openly gay opponent.

She also had some advice for Weiner's wife, Huma -- "Whatever his wife does is her decision, but I would leave him."

A little audacious, don't you think?