“The most polarized part of a polarized state in a polarized nation” — that’s how Craig Gilbert of The Milwaukee Journal Sentinel describes the Milwaukee metropolitan area in the masterful first article of a four-part series on the region’s politics. The region is just as polarized as Mr. Gilbert suggests.

Milwaukee is not the only metropolitan area divided between a heavily Democratic, nonwhite urban core and overwhelmingly white suburbs. But the Milwaukee suburbs are still unusual. They haven’t moved at all toward the left, unlike other Republican-leaning Northern suburbs.

In the Reagan era, nearly every white suburban county outside the South voted overwhelmingly Republican. The Milwaukee area was no different. Democrats earned the same share of the vote in Waukesha County, Milwaukee’s largest Republican suburb, as in collar counties (those that surround cities) across the Northeast and Midwest. If we had precinct-level data from the 1980s, the maps of Philadelphia or Detroit would look just like the Journal Sentinel’s gorgeous but starkly polarized maps of Southeast Wisconsin.

Democrats have made huge gains in the suburbs since the Reagan years. Bill Clinton defused old wedge issues by embracing welfare reform and getting tough on crime. Macomb County, home to the much-discussed Reagan Democrats outside Detroit, swung decidedly toward the Democrats. Demographic change offered additional help to Democrats in many suburban counties, but Democrats made big gains in the whitest suburban counties as well.