I know a few people have experienced this and reported it as a bug. here is the explanation, from looking at someone’s save game:
Check your cabinet, and their campaigning skill. The likelihood is that all your ministers skill is really low This is the reason that your high approval does not translate into decent
party membership. Basically people love your policies but hate all the
main people in your party 
If you think this is too harsh, its governed by a value in \program files\democracy 3\data\simconfig.txt called:
ELECT_MINISTERS_TOTAL_IMPACT
If you reduce that, it wont be so strong, but thats almost cheating
Hope that explains things