It seem that in most urbanized areas in recent years (2000 - present) Democrats win more elections on average than Republicans. According to debt.org:
if an area has more than 500,000 people, it will go Democrat about 60 percent of the time. The political balance is even in areas with 50,000 to 500,000 people, and then turns decidedly in the Republicans’ favor at population levels below 50,000.
According to this website areas with higher population density tend to vote democrat,
At about 800 people per square mile, people switch from voting primarily Republican to voting primarily Democratic. Put another way, below 800 people per square mile, there is a 66% chance that you voted Republican. Above 800 people per square mile, there is a 66% chance that you voted Democrat. A 66% preference is a clear, dominant majority.
During the 2012 elections, this happened:
Of the 15 largest U.S. cities, 11 went for Obama this past November. Not just the big, traditionally blue cities of the Northeast, Midwest, and West Coast, but a number of fast-growing Sun Belt cities as well.
Additionally, the GOP themselves recognize their problem with urbanized areas.
What factors have inhibited Republican success in these areas?
What factors account for the Democrats success in these areas?