Career politicians like Pelosi are Democrats just by chance. While at College she interned for a Democratic senator because her father was a Democratic congressman which got her into that position and therefore got her career path moulded in the Democratic party. Nothing else to it. Just a 50:50 chance, could have also interned for a Republican senator and would be a Republican now.
She is the type of politician who is in it for her own career, she has no ideas nor ideals.
The highest ranking Democrat in the country is a girls-only private Catholic college graduate who is married to a banker, favoured waterboarding, supported war in Libya etc.
America is full politicians like this. They have the choice between Republicans and Republicans who find smoking pot cool (aka Democrats).