Question: When early voting is created or extended what impact does it have on political races?

I was looking at the Presidential Commission on Election Administration report and noted their discussion on early voting.  Just some thoughts: most campaigns, particularly those of challengers, try to wait until near the election before they spend their limited funds on advertising.  What impact does early voting have on how well informed voters are?  Does early voting increase the re-election rate for incumbents?  Does it change the rate that Democrats or Republicans win election?