Every four years, the electorate gets younger. Most young voters are openminded and see no reason to discriminate against anyone based on race, sex, or gender preference. They replace older voters, who generally are less open to change. Here's an article that shows just how Democratic young voters are:
The article also shows that women, African-American voters, and Hispanic voters are much more Democratic than white men. These groups were the key to President Obama's success in 2008 and will be again this year.