(idiomatic) A candidate or contestant generally agreed upon as the presumptive winner; somebody who is well-liked or widely agreed upon.
Example: 1895, â€œPointers from the Paddockâ€ , The San Francisco Call
Little Pete was a member of the delegation that were in on the "shoo" in the third race. He had several commissioners placing his coin on Model. If the race was to have been a "shoo-in" for Model, there must have been one or two owners that were not in on the deal.
1910, â€œVernacular of the Race Trackâ€ , New-York Tribune
Most of the "regulars" are deeply suspicious of all steeplechase races of late years, and, whenever the favorite falls at one of the obstacles and a long priced leaper wins the race, they loudly call the race a "shoo-in" (a fixed affair, that is, in which the steeplechase racers have arranged to drop to the rear of the "meant" jumper and "shoo" him to the wire, they previously, of course, having got their money down on the horse thus generously treated).
1950, Baltimore Sun
[it was] wondered whether Chris Chenery's Virginia flyer would be a shoo-in for the Belmont Stakes …
She's very popular and good with numbers, so I expect she'll be a shoo-in for treasurer.
2007, Governors Speak, by Jack D. Fleer, page 47:
Participation in primaries was often more important than in the general election because the Democratic Party nominee was a shoo-in for success in the general election.