I have a large collection of resources at The Best Sites To Learn About U.S. Presidential Elections. I put it together for the 2008 Presidential election, but about half of the links there would be very relevant to this year’s election, too.
I probably won’t put together a new version of that list until the Republicans decide on a presidential candidate. In the meantime, though, there are plenty of good sites there.
You might also want to check out Scholastic’s Election 2012 website.