The tumultuous 2016 race is on track to deepen both of these trends. With Donald Trump facing widespread rejection by minority voters and underperforming literally every Republican nominee in the history of modern polling among college-educated white voters, he faces the risk of voters further retreating from the party in the most densely populated metropolitan areas across the swing states.

But with his continued support among white blue-collar and non-urban voters, he may hold, or even expand, the Republican advantage beyond the metros. This history suggests the formula for Hillary Clinton is to run up the score in the most populous places. Trump’s hope in these states rests more on death by a thousand cuts — small increases in margin across many smaller counties that overcome erosion in the metro areas. That was largely the formula that allowed Romney to tip back North Carolina in 2012 despite persistent deficits in the biggest places.

The maps below capture these changes. For Ohio, Florida and Colorado, they measure the increase in the total number of Democratic votes by county from 2004 to 2012. In each case, the map divides the states into three bands: counties where the Democratic increased their total vote by more than the state average; counties where Democrats saw an increase but one that was less than the state average; and counties where the Democrats won fewer votes. In North Carolina, the map tracks the same changes for the Democratic vote from 2008 to 2012.

Ohio

Florida

Colorado

North Carolina

Despite these states’ many differences, these results show some strikingly similar patterns. One is that Democrats have relied overwhelmingly on the most populous places for their gains. In Ohio and North Carolina, over half of Democratic gains came from their improvement in just the three most populous counties; in Florida and Colorado, half of the gains came from just the five largest counties.

In Ohio, the state’s most populous counties — namely those surrounding Columbus and Cincinnati — saw vast increases in Democratic votes that overwhelmed the losses elsewhere between John Kerry’s defeat in 2004 and President Obama’s victory in 2012. Democrats enjoyed their biggest increase in Franklin County of Columbus: it provided 60,000 more votes for Obama in 2012 than it did for Kerry.

Hamilton County, which includes Cincinnati, provided 20,000 more votes for Democrats, ultimately swinging from red in 2004 to blue in 2012. And while Cuyahoga County in Cleveland was an exception, with a decrease of almost 1,300 Democratic votes, the county still provided the Democrats with the most votes in the state in 2012, nearly 450,000 votes—and that Democratic loss of 1,300 votes was minor compared to the GOP’s loss of 31,000 votes in that county in the same year. In all, the three Ohio counties with the greatest increases in Democratic votes from 2004 to 2012 contributed an additional 90,000 votes to the Democratic Party. That surge overwhelmed smaller counties where Democratic support declined.