a political opportunist
who changes ideology with each new opinion poll and survey. A turncoat will betray his principles, if he had any, admit to having lied about previously stated principles and in general will say anything to get elected. Usually turncoats are serial politicians whose primary goal is to further their own political career.
Turncoat Florida Governor Charlie Crist, whose vertiginous politics change with the wind, in early 2010 changed his stripes from solidly lifelong conservative Republican, to free-thinking independent, now to Democrat all in the span of a few months.