Well something that works for me alot is setting up campaign headquarters/outreach offices in states in the west when I am republican, and when I am a democrat do the same for the east.  These buildings raise your awareness in these states, and if you do this early, you will have an early advantage over your oponent.

Make sure to do fundraising in California, Texas, Ohio, Pennsylvania New York, and Florida when you can. Also try and win these states since they have the most electorial votes. Don't try and just win them, but make sure you visit them often with speeches, political opreatives and ads to keep polls leaning for you.

Make sure to be on the look out for little cameras on the map, the one that is the color of your party is the one you want to travel to.  You will have an interview, if you do well you can swing the independant vote of the whole nation towards you, if you do badly, you may lose independant voters.

After you play for a while, losing and/or winning you will pick up on things and it will be easy.  Try playing quick mode on like a low difficulty, then raise it up as you win.