The problem with using stacked bars in political situations is that the 50% threshold is critical. Without reading the numbers, it's much harder to tell with stacked bars whether a close vote (or poll) is at 48% or 52%.
You don't need to go back to a pie chart for that. On the chart above that I created there is a clear 50% line in the middle. If necessary, it could be darkened to make the mid point more obvious.
