But in the nation’s smaller metro areas, where the share of suburban House seats held by Republicans rose to 71 percent after 2018 from 60 percent after the 1994 election, Republicans continue to thrive. Even last November’s “blue wave” hardly threatened Republican incumbents like Warren Davidson of suburban Cincinnati-Dayton, who won re-election by 33 percentage points; Gary Palmer of suburban Birmingham, Ala., who won by 38 points; or Francis Rooney of suburban Cape Coral, Fla., who won by 25 points.

Uneven patterns of racial diversification explain much of this growing partisan gap. In the top 20 metro areas, more than a third of adult suburban residents are now nonwhite, compared with less than one-quarter of the suburban population in smaller metros. The number of large-metro suburban House seats where members of racial minorities outnumber whites increased to 36 in today’s Congress from nine in 1993 and is poised to grow further after the national round of redistricting following the 2020 census. White voters in large metros are also politically distinctive, holding relatively liberal cultural views that make them more likely than whites elsewhere to support Democratic candidates.

While pundits once advised Democratic leaders that courting suburban support would require breaking with liberal orthodoxy, the party’s new electoral coalition of minorities and culturally progressive whites has allowed it to unite around left-of-center positions on issues from gun control to gay rights.

Still, internal tensions remain. House Democrats elected from majority-white suburban districts are nearly twice as likely to join the centrist New Democrat Coalition or Blue Dog Coalition as they are the Congressional Progressive Caucus. And some express concern that the recent publicity received by left-wing policy proposals like the Green New Deal will threaten the party’s popularity among their more economically moderate constituents.

The increasing Democratic strength within populous, racially diverse metropolitan areas like Atlanta, Houston and Phoenix will leave its mark on the electoral map as soon as the 2020 election, transforming Georgia, Texas and Arizona from Republican bastions to contested battlegrounds. A few other metro areas that are slightly smaller in size but comparable in demographic composition, like Las Vegas; Richmond, Va.; and Raleigh-Durham, N.C., are also likely to become increasingly critical sources of Democratic votes in national elections.

But President Trump’s historically strong performance in a string of smaller and more homogeneous suburbs from greater Scranton, Pa., to greater Des Moines proved pivotal in the 2016 election and could well recur in 2020. Broadening the Democratic tent to bring more of these socially traditionalist small-metro suburbanites into the fold would provide the party with a critical electoral advantage, but such gains will be difficult to achieve in an era of growing cultural warfare.

Rather than moving in a single partisan direction, American suburbia — like the nation of which it is a growing part — has split into incompatible halves of brightening red and deepening blue, frustrating both parties’ attempts to build a stable national majority on a foundation of suburban votes.