Social and political scientists have plenty of evidence that Americans have spent the past decades sorting themselves politically.

There's also evidence that living among like-minded thinkers may be helping our preferred political parties in some elections -- and hurting it in others. If Democrats are concentrated in urban areas, they have less of a say in the makeup of Congress, yet more of a say in the presidential race. (While there are other factors at play, it's no coincidence that for the past six years, we've had a Republican-controlled House of Representatives with historically big majorities and a Democratic-controlled White House.)

A new Pew Research Center survey puts new numbers to just how concentrated we are politically and what that means at the ballot box. Researchers found America's geographic sorting by politics drastically shifted the political makeup of America's biggest cities in favor of Democrats.

Of the 83 counties that were among the most populous in both 1976 and 2012, they found more than half (46 counties) shifted Democratic by a significant margin.

Many of these once-competitive counties now favor Democrats by more than 20 percentage points. Only nine of the 83 counties surveyed became less Democratic to any degree, writes Pew senior researcher Drew DeSilver.

Here's another way of looking at it: Since the 1988 election, these counties have consistently voted for the Democratic candidate for president, and by bigger and bigger margins each time.

This urban shift to blue is great news for someone like Hillary Clinton. A higher percentage of voters are concentrated in bluer and bluer areas. As Fix Boss Chris Cillizza recently pointed out, the electoral map favors Clinton before the general election race has even begun: There are 17 states (and the District of Columbia) that have voted for the Democratic presidential nominee in each of the six presidential elections between 1992 and 2012. That's in part because Democrats (including nonwhite voters, which lean Democratic) are concentrated there.

But it's bad news for Democrats trying to use Donald Trump's unpopularity to chip away at or even erase Republicans' historic majority in the House of Representatives. Because Democrats are clustered in one area of the state, they have less of a say in who represents congressional (and state-level) districts in the rest of the state.

During the Obama years, we've watch Republicans earn their biggest post-World War II majority in the U.S. House. They also control 69 of 99 partisan state legislative chambers, and they control 31 of the 50 governor's mansions.

There are other political factors contributing to our nation's divided political makeup, of course. Who's in the White House often benefits the other party in congressional and state elections.  But this Pew Research report makes clear that where we live is a major one.

Political clustering helps create congressional makeup like Nevada's. Of the swing state's four congressional districts, only one is currently represented by a Democrat — Las Vegas, which also happens to be Nevada's largest urban center by far. The rest of the mostly rural state is represented by Republicans. (Though some of the seats surrounding Las Vegas could change parties this November.)

This data frames how politicians try to win elections, too, and perhaps who they cater to once they win said election. A Republican candidate running for statewide office is probably not going to bother much with the urban part of her state because the data shows she can't win it.

Meanwhile, a Democratic candidate running for statewide office is less concerned with the rural parts of his state. They two candidates will be speaking to two different audiences when they campaign, and all else being equal, who wins comes down to which of those audiences come out to vote in higher numbers.

This has the potential to play out after Election Day, too. If you're a politician who won thanks to rural voters, it makes sense they'd be your first priority when you get to office.

Partisanship, divided government, politicians loyal to certain factions. It isn't new. But the geography of those divisions has never before been quite as stark.

What the scene in Philadelphia looks like as it readies for the DNC

PHILADELPHIA, PA - JULY 24: A Bernie cut-out is brought by attendees during "The Climate Revolution Is Up To Us" rally at Vernon Park on the eve of the Democratic National Convention on July 24, 2016 in Philadelphia, Pa. (Ricky Carioti/The Washington Post)