I just googled urban vs. rural in the US. Based on the 2010 census 81% of the population is urban. The balance is rural which makes up 72% of the land area, but sparsely populated.
Romney won 78% of the counties in the US, but as we know, Obama cleaned up in the urban areas. That's why he's president.
All you have to do is look at the 2012 election map by county and see the sea of red and you immediately make the connection that most of those red counties are rural and in the larger states are heavily outvoted as a whole by one or two urban areas in their state.
I just looked into it further and discovered the US became urban majority between 1910 and 1920 - 100 years ago! Not sure why you think it is 50/50. Look around you a little bit more.
Go to the Virginia state board of elections web site and look at the registration by county. You will be surprised. Fairfax County has over 650,000 voters registered. Bath County and a ton of others have just a handful of voters.
Take the population of the 50 biggest cities. Then divide by the total US population. You will get .5.