For the latest evidence of the nation’s polarized politics, the granular returns from November offer these slivers of bright purple insight:

Voters in just 35 congressional districts, or 8 percent of the total, elected a House member from one party while preferring the presidential candidate of the other party — the second election in a row where the share of ticket-splitting seats was in the single digits. Before that, 1920 was the last time the number of such crossover districts fell below one out of every nine.

The voters’ monolithic behavior was even more historic in the results for the Senate. For the first time since 1916 — which was the second election with senators chosen by popular vote — not a single state divided its political preferences. Republicans won 22 contests, all in states carried by President Donald Trump. Democrats won the other dozen races, each in a state where Hillary Clinton prevailed.

The statistics are the latest, compellingly quick way to illustrate how dramatically the nation has divided itself into clearly delineated pockets of ruby red and navy blue — and how the places where the partisan true colors bleed together have become fewer and farther between.

But beyond that, the results offer an early window into what the potentially small and spotty battlefield will look like for the already-getting-started midterm campaign, when Democrats are hoping a wave of Trump buyer’s remorse propels them out of one of their worst positions on the Hill since the 1920s.

It’s the purple places where the party out of power always has its most obvious targets of opportunity, even if no underlying wave materializes for more sweeping change.

But because of the dramatically shrunken roster of obvious swing districts, which by definition are under the thrall of independent-minded voters, the 2018 election will probably be another battle that’s less about persuading the middle than about which side is better at motivating its base to turn out.

A narrow path

At first blush, it might seem like a partisan color wash at the Senate level, because Trump carried 17 states conducting contests in 2018 while Clinton carried 16. But that’s not the best way to look at it.

Instead, consider how the GOP will be defending seats in only eight states, which all went for Trump except Dean Heller’s Nevada. But 25 Democratic caucus members are up for re-election — a consequence of the six-year senatorial cycle and the party’s strong showings in both 2006 and 2012.

And not only do 10 of those Democrats represent Trump states, but also half of that group are from states that turned red last time after twice falling in Barack Obama’s column: Bill Nelson of Florida, Bob Casey of Pennsylvania, Sherrod Brown of Ohio, Debbie Stabenow of Michigan and Tammy Baldwin of Wisconsin.

Those numbers alone suggest how extraordinarily difficult it looks for the Democrats to score the net gain of three seats they’d need to take back the Senate, no matter how antagonistic the electorate might become toward the Trump administration and his partisan helpmates running Congress, who are just now getting started running the first GOP government in a decade.

That’s why some Democratic strategists are having preliminary discussions about throwing all the party’s financial and organizational weight behind an attempt to take over the House, a goal widely discounted throughout this decade as virtually impossible.

For 2018, it would require a net gain of two-dozen seats. And the presidential results for every congressional district  — as calculated by Daily Kos Elections — suggest a path for the Democrats getting tantalizingly close:

Start by targeting the 23 seats currently occupied by a Republican even though their voters preferred Clinton for president. And work fervently not to relinquish any of the 12 districts that voted in November for both a Democratic representative and the presidential winner.

The good news for Democrats is that the map of crossover districts where both Clinton and GOP House members prevailed is larded with places that have not produced such split-ticket results in many years — mostly suburban areas where the populations are becoming younger and more ethnically diverse.

These results suggest that Clinton’s effort to expand her party’s playing field — which helped generate her 2.9-million popular vote margin but was not sufficient for an Electoral College win — nonetheless have marked a trail for Democratic inroads in the elections ahead.

That, of course, assumes her party can find and fund viable candidates. Democrats essentially did not contest the re-elections of five Republicans representing territory that went Democratic for president for the first time in at least two decades: Ed Royce, Mimi Walters and Dana Rohrabacher, whose districts form a crescent through Orange County east of Los Angeles; and Texans John Culberson of Houston and Pete Sessions of Dallas, whose ethnically diversifying districts were the provinces of the Anglo business elite until relatively recently.  

Other Republican incumbents in recently safe seats saw their suburban constituents revert to their Democratic presidential preferences of 2008 after backing Mitt Romney four years ago: Ryan A. Costello and Patrick Meehan west of Philadelphia, Leonard Lance in northern New Jersey and Peter Roskam outside Chicago.

The other Republicans who prevailed in Clinton districts are familiar names to those familiar with three relatively short rosters: Members in districts drawn to give neither side any edge in the partisan base vote (Martha McSally of Arizona, Will Hurd of Texas and Carlos Curbelo of Florida, for example), lawmakers who were targeted unusually intensely last year (among them Darrell Issa of California and Kevin Yoder of Kansas) and those who have handily survived several straight elections despite the top of the ticket going the other way (including Ileana Ros-Lehtinen of Florida, Dave Reichert of Washington and Erik Paulsen of Minnesota.)

Trump’s small-map impact

Even if they pick up all those “GOP-Clinton” seats, and hold all their own “Democratic-Trump” seats, to become the majority again, Democrats would still need to take over a few of the 218 districts that voted Republican for president as well as Congress last fall.

One surprising aspect of the results is what they do not reveal: While Trump won the presidency because he narrowly captured a trio of Great Lakes states — Wisconsin, Michigan and Pennsylvania — that Democrats had assumed were part of their “Blue Wall,” his confounding of that conventional political wisdom did not extend to a ticket-splitting scramble at the congressional level.

In fact, on paper anyway, only six districts across the country look like they reflected the singularity of his formula for success, because he won these stretches of rural America even though Obama had carried them by substantial margins in both of his elections — and even as they returned Democratic incumbents to the House: Sean Patrick Maloney in New York’s Hudson Valley, Matt Cartwright in eastern Pennsylvania, Cheri Bustos in northwestern Illinois, Dave Loebsack across the Mississippi River in northeastern Iowa, Ron Kind in southwestern Wisconsin and Rick Nolan in the vast Iron range region of northeastern Minnesota.

Five of the other Trump-Democratic districts, most with a suburban cast, have been electoral tossups for years. The sixth, in western Minnesota, is where veteran conservative Democrat Collin C. Peterson has maintained a steady hold even though it’s reliably Republican at the presidential level.

How the splits went away

The handful of longtime survivors in crossover territory used to have dozens of colleagues as their company.  

The height of split-ticket voting was two generations ago, coinciding with the closing of the era when conservative Southern Democrats held the balance of power on the Hill. In two of the biggest presidential landslides ever, Richard Nixon’s 1972 re-election and Ronald Reagan’s second term victory in 1984, divided government continued because both times 44 percent of districts nationwide (a huge bloc of them in the South) voted Democrat for Congress and Republican for president.

But even in the extraordinarily close presidential contest of 2000, and as recently as Barack Obama’s election in 2008, voters in 19 percent of districts split their tickets — with 49 Democrats victorious in districts that went for John McCain and 34 Republicans winning territory carried by the presidential victor.

The number shriveled to a record-low 6 percent (26 districts) when Obama was elected four years later, by which point the South had become as Republican as it was once Democratic, and the split-district map edged up by only nine districts last fall — a sustained consequence of a confluence of interconnected trends and decisions.

The tea-party wave election of 2010 returned the Republicans to power largely at the expense of conservative “Blue Dog” Democrats, dozens of whom were swept from office.  

The redrawing of congressional lines for this decade was dominated by Republicans in state legislatures, but they and the Democrats generally went along with maps that protected the new incumbents and guaranteed electoral safety for one party or the other.

This was made easier by an ever-more-mobile population sorting itself into ideological neighborhoods as never before, with liberals flocking to the cities and close-in suburbs and conservatives choosing the exurbs and rural areas.

Amazingly, of the 49 Democrats who won in McCain districts just eight years ago, Minnesota’s Peterson is the only one who’s still a member of Congress today.