The fact that there are more Democrats than Republicans doesn't guarantee that Democrats will always win. In midterm elections, Democrats are less likely to vote than Republicans. That's why Republicans did so well in the 2010 and 2014 elections. In presidential elections, however, Democrats have outnumbered Republicans in every contest since 1992 except 2004 when there was a tie. That's the main reason Democrats have won the popular vote in every presidential election since 1992 except 2004.