Of course users will feel free to answer, but you might start with the section of the subreddit wiki that directly addresses this question. If you have questions for any of the users who answered in these threads, you can always ping them using the /u/username format.

In particular, you will be interested in these threads:

http://www.reddit.com/r/AskHistorians/comments/yczua/can_someone_address_a_brief_history_of_democrats/

http://www.reddit.com/r/AskHistorians/comments/12vvo4/how_and_when_did_the_republican_party_transition/

http://www.reddit.com/r/AskHistorians/comments/12glzr/what_changes_occurred_in_the_last_century_that/