There are three main goals to political marketing, enlisting volunteers, raising money and getting your message out and voted for. Its easy enough to set up conversion goals in an analytics system for volunteer signups and donations (ROI calculations are even possible for the donations), but measuring message acceptance can get tricky short of doing a poll or survey type questionaire. Traditional engagement metrics like time on site and page views per visit as well as loyalty metrics like return frequency and percentage of return visitors and be used to measure can be an approximation. I’d expect to see an “agreement” metric correlate to both donations and volunteer signups. Those three metric groups are the best KPI for any multivariate testing, which is also a very powerful political marketing technique for the web.