A friend just emailed me that a particular election was a "shoe-in." I think it should be "shoo-in" but not sure. And it made me curious about where the phrase comes from. Anyone know?

it's shoo-in, from the interjection 'shoo!', and from horse-racing argot.

1908 G. E. SMITH Racing Maxims & Methods of ‘Pittsburgh Phil’ ix. 123 There were many times presumably that ‘Tod’ would win through such manipulations, being ‘shooed in’, as it were. [OED online]