How Well Do You Know English Slang?
"easy winner" (especially in politics)," 1939, from earlier sense "horse that wins a race by pre-arrangement" (1928); the verb phrase shoo in in this sense is from 1908; from shoo (v.) + in (adv.).
: to be a shoe-in candidate
[shoe-in may simply be a misspelling, but it is probably based on a misunderstanding of the origin of the term; it is a sort of folk etymology]