Long-shot

a horse, team, etc., that has little chance of winning and carries long odds.
an attempt or undertaking that offers much but in which there is little chance for success.
Movies, Television. a camera shot taken at a relatively great distance from the subject and permitting a broad view of a scene.
Compare closeup (def 2), medium shot.
by a long shot, by any means; by a measurable degree:
They haven’t finished by a long shot.
Contemporary Examples

Tea Party Reindeer Farmer Faces Extinction Ben Jacobs July 29, 2014
Mitt Romney, Boring Genius? Howard Kurtz June 26, 2011
Danger Signs for Mitt Romney as Colorado, Minnesota, Missouri Ready Vote Howard Kurtz February 6, 2012
The Last Vietnam War Epic John Douglas Marshall April 7, 2010
Hermain Cain: The Pizza Man Delivers Michael Medved May 5, 2011
‘Blow Your Balls Off’ Senate Candidate Wants to Give Away a Free Car Olivia Nuzzi April 17, 2014
2016 Dark Horse Martin O’Malley Is Boosting Iowa Democrats (and Himself) Ben Jacobs September 21, 2014
What If It’s Not Hillary? Meet the Democrats’ Plan B Team for 2016 Ben Jacobs December 1, 2013

Historical Examples

The Debtor Mary E. Wilkins Freeman
Punchinello, Vol. 1, No. 13, June 25, 1870 Various

noun
a competitor, as in a race, considered to be unlikely to win
a bet against heavy odds
an undertaking, guess, or possibility with little chance of success
(films, television) a shot where the camera is or appears to be distant from the object to be photographed
by a long shot, by any means: he still hasn’t finished by a long shot
n.

A person, horse, project, etc, that seems not likely to win or succeed; dark horse: But it’s a pretty long shot I’m afraid (1869+)
A scene photographed from a distance; a long-range photograph (1940s+)