snatch defeat from the jaws of victory
To fail, lose, or be defeated despite the appearance that one would be victorious, especially due to a mistake, error, or poor judgment. (An ironic reversal of the more common "snatch victory from the jaws of defeat.") We were ahead by nearly 20 points with less than half the quarter remaining—how on earth did we manage to snatch defeat from the jaws of victory like that? The candidate has led in the polls right up to election day, but with that unfortunate remark last night, he may well have snatched defeat from the jaws of victory.