This website is quite good if you want a definition plus the history of the phrase: http://www.phrases.org.uk/meanings/queer-the-pitch.html

It sounds like it is referring to two candidates campaigning for an election. A "pitch" could either be the speech they give (or the more abstract ideas about themselves that they spread) to promote themselves, or a baseball pitch (a throw by the pitcher) being used as a metaphor. In the first case, everyone thought candidate A would win, but when the well-supported candidate B appeared, candidate A's pitch no longer sounded so good. In the second case, the candidate is predicted to win like a straight-flying baseball pitch, but the ball was 'queered', meaning that it was pushed off course and no longer a good pitch. In this case, the pitch is just a metaphor for the candidate's election campaign.