Figuratively, to salvage something is to save it from disaster. Thus, to salvage victory is to obtain a narrow victory after having been facing defeat.
Example: (Note: Strictly opinion of original writer) U.S. Chamber of Commerce president Tom Donahue, writing about the Democratic Party's attacks on his organization: “It’s sad to watch the White House stoop to these depths to try to salvage an election,” Donohue wrote, according to The Times’ Michael D. Shear.
This means, the attacks on his organization are meant to rescue the Democrats from widely predicted losses in the mid-term elections. This usage - in politics and in general - is common in English.