Ever since the New Deal, the Democratic Party has been known as the party of big cities. Perhaps the most basic fact of American politics is the divide between Democratic cities and Republican rural areas.

This has been true regardless of the relative strength and weakness of the two parties. When Ronald Reagan was winning landslide victories in the 1980s, he couldn’t win, or even come close, in major northern cities like New York, Chicago, Boston, Philadelphia and the like.

However, just in the past decade the Democratic Party has become appreciably stronger in the largest urban areas. This can be observed by comparing the 2004 and 2012 presidential election results.

George W. Bush ran 3 points ahead of Mitt Romney in the national popular vote, but the difference was not distributed evenly across urban and non-urban areas. In the top 100 most-populated counties measured by the 2010 Census, Bush received 43 percent of the vote.

Romney ran 5 points behind, earning 38 percent. In the other 60 percent of the country that’s not part of the 100 most-populated counties, Romney only ran 1.5 points behind Bush. If Romney had been able to swing the top 100 counties as much as he did the areas outside them, the popular vote would have been within 1 point and a tossup in the Electoral College.

The changes in select counties have been swift. In 2000, Bush and Gore were within a few points of each other in the following counties: Franklin County, Ohio (Columbus); Fairfax County, Virginia; Mecklenburg County, North Carolina (Charlotte); and Marion County, Indiana (Indianapolis).

In 2012, Obama won Franklin County by 23 points, Fairfax County by 20 points, Mecklenburg County by 22 points and Marion County by 22 points. In all of these counties, Romney ran about 10 points behind Bush 2000 numbers. These rapid changes explain, in part, how Ohio and Virginia have gone for Obama twice and how North Carolina has become a presidential swing state.

If we look back to the 1980s, the change in urban areas is even more stunning. In 1988, George H.W. Bush, a candidate very similar to Mitt Romney in background and style, won 47 percent in Los Angeles County, 43 percent in Cook County (Chicago), 40 percent in Queens, 33 percent in Philadelphia, 40 percent in Essex County, New Jersey (Newark), and 46 percent in Hudson County, New Jersey (Jersey City).

In 2012, Romney received 28 percent in Los Angeles County, 25 percent in Cook County, 20 percent in Queens, 14 percent in Philadelphia, 22 percent in Essex County, and 22 percent in Hudson County. Bush did not win any of these counties, but the difference between losing Philadelphia by a 2-to-1 margin and losing it by a 7-to-1 margin is the difference between a party with a winning presidential coalition and a party that is continually shut out of the White House.

There are similar long-term declines in many heavily populated suburban counties. In 1988, Bush ran at 68 percent in Orange County, California, 61 percent in Oakland County, Michigan, 53 percent in Westchester County, New York, 58 percent in Bergen County, New Jersey, 59 percent in Fairfield County, Connecticut, 60 percent in Montgomery County, Pennsylvania, and 61 percent in Fairfax County, Virginia.

This year, Romney hit 52 percent in Orange County, 45 percent in Oakland County, 37 percent in Westchester County, 44 percent in Bergen County, 44 percent in Fairfield County, 42 percent in Montgomery County, and 38 percent in Fairfax County. Whereas Republicans could once count on these counties to offset Democratic margins in nearby central cities, now they often add to the deficit incurred in metropolitan areas.

In most of these states, there aren’t enough rural voters to make up for the one-two punch of monolithic Democratic cities and marginal-at-best suburban counties. It is clear from these numbers that the coalition that elected Reagan twice no longer exists.

For a whole host of reasons, demographic changes being the most conspicuous, the American electorate has changed. These changes have been the most concentrated in the largest urban areas of the country, and they have changed the United States from a country where on balance it’s easier to elect a Republican president to a country where it’s easier to elect a Democrat president.

Chris Palko works as an assistant media analyst at Smart Media Group, a Republican political media buying agency in Alexandria, Va. He is a graduate of American University and George Washington University’s Graduate School of Political Management.

A version of this post was also published on Smart Media Group’s blog, Smart Blog.