1) Best bet 2) Certain victor 3) Certain winner 4) Cinch to win 5) Easy winner 6) Election day certainty 7) Great bet 8) Guaranteed winner 9) Lock 10) Lock, so to speak 11) More than a favorite 12) No-doubt winner 13) Obvious winner 14) One with a good poll position 15) Slam dunk, so to speak 16) Sure thing 1) Chases through the door 
• a candidate who is certain to win easily.
Shoo-in is American slang for a certainty.