dark horse

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One who achieves unexpected support and success as a political candidate, typically during a party's convention.
• noun A little-known, unexpectedly successful entrant, as in a horserace.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic, politics A candidate who is nominated unexpectedly, without previously having been discussed or considered as a likely choice.
• noun rare An unexpected success.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a political candidate who is not well known but could win unexpectedly
• noun a racehorse about which little is known