"shy tory"
In the US we speak of "voting-booth candidates". Years ago, Jesse Helms was a couple of times polled as losing his senate seat from North Carolina, but then won handily. Then Marion Barry, who had been videotaped consuming crack cocaine, was elected mayor of Washington, DC, despite polls showing him doing badly. In both cases the voters polled presumably answered according to the tone of the question rather than according to their true intentions.
Biting the hand that feeds IT
