2018 rewrote the main rule of US politics

The results of last week's election demonstrated how powerfully national trends now shape election outcomes in every region. The election produced remarkably consistent divides along demographic and geographic lines in states as diverse as Arizona, Georgia and Texas on one side, and Michigan, Wisconsin and Pennsylvania on the other. Though some important regional differences remained, voters who shared the same characteristics or resided in similar places largely voted the same way no matter what state they lived in.

In virtually every state, Democrats last Tuesday displayed a clear advantage in densely populated, culturally and racially diverse white-collar metropolitan areas, while Republicans relied on elevated margins in the preponderantly white, religiously traditional, smaller places beyond them. In almost all cases, the outcome in each state was determined less by how much they varied from that persistent pattern than by how much of each group was present in the state's electorate to begin with.

The continued nationalization of American politics threatens greater polarization and social tension as the lines harden between these two distinct political coalitions. But paradoxically, it means that the party that can generate the most exceptions to these solidifying trends may be the one most likely to control Congress and the White House.

Whether measured by demography or geography, the 2018 election produced remarkably consistent patterns.

A surprise twist among older voters

Viewed by age, the results offered a vivid portrait of the price Republicans are paying among younger voters as Trump redefines the party in his confrontational image.

The exit poll measuring preferences in House elections found that Democrats carried fully two thirds of voters aged 18-29. That was their best showing with them in exit polls since at least 1986 (narrowly exceeding their level even in former President Barack Obama's sweeping 2008 victory) and a big improvement on Hillary Clinton's 55% among them in 2016. And preliminary calculations indicate that youth turnout may have been half again as large in 2018 as it was in 2014, the most recent midterm.

Even more striking was the consistency of the Democratic advantage around the nation. The Democratic candidate won voters aged 18-29 in all 21 Senate races with an exit poll except for Indiana, where Joe Donnelly tied Republican Mike Braun. (These figures do not include the exit poll in California, where two Democrats ran against each other after claiming the top spots in last June's state top-two primary.) Senate Democrats carried about three-fifths or more of these younger voters in Florida, Michigan, Minnesota (both for incumbent Sen. Amy Klobuchar and Tina Smith, who was elected in a special election), Montana, Nevada, New Jersey, New York, Pennsylvania, Tennessee, Texas, Virginia, West Virginia and Wisconsin. Democrats also reached at least 60% with them in governor's races in Florida, Georgia, Michigan, Minnesota, Nevada, Wisconsin and Pennsylvania. In the California governor's race, Democrat Gavin Newsom carried 69% of younger voters.

Just as important, the Democratic advantage extended up the age ladder. Against Trump in 2016, Clinton carried only 51% of voters aged 30-44; while Trump won just 41% of them, a substantial 8% scattered to third-party candidates.

This time, those voters consolidated behind Democrats. In the national House exit poll, Democrats won 58%. The margins weren't always as large in the Senate races with exit polls. But every Democratic Senate candidate carried the voters in this age group except for Heidi Heitkamp in North Dakota. In states from Pennsylvania, Minnesota (both races) and Wisconsin to Florida and Nevada, Democrats carried about three-fifths of these voters. (The Arizona exit poll results combined voters aged 18-44: Democrat Kyrsten Sinema carried 59% of the combined group.)

Republicans in turn consistently performed better with older voters -- though with a surprising crack in their armor. In the national House exit poll, voters aged 45-64 split almost exactly evenly between the two parties. But Republicans won them, often convincingly, in most of the closely contested Senate races, including Florida, Indiana, Missouri, Montana, Nevada, North Dakota, Tennessee, Texas, and West Virginia. That group of older workers, often described as the "anxious generation" facing the final years before retirement, has emerged as a key Republican constituency.

The results among seniors, though, were competitive. About four-fifths of today's seniors are white and they have moved right in recent elections: Trump won them in 2016, and Republicans have consistently carried them in recent congressional contests, exit polls have found. But this year they split about evenly in the national House exit poll (48% for Democrats versus 50% for Republicans) and in the state races they scattered more than most groups.

Republicans won them convincingly in the contested Senate races in Florida, Tennessee, Indiana, Missouri, Texas and Nevada; but Democrats won them with solid margins in Ohio, Pennsylvania, and Michigan, carried them narrowly in North Dakota and West Virginia and essentially split them in Montana and Wisconsin. (In the tight Arizona Senate race, Republican Martha McSally comfortably carried voters in the combined category of 45 and older.)

The racial and educational divide grows

Across the country, the results converged along educational lines too.

In the national House exit poll, Republicans carried 61% of whites without a college education, Trump's best group. It's difficult to precisely compare that to previous years, because the exit polls changed the way they weight the results for education. What is clear from last week's results is the Republican advantage with these voters persisted in all regions: Klobuchar in Minnesota was the only Democratic Senate candidate to win most white voters without a college education, although Tester and Manchin came close.

Both Joe Donnelly in Indiana and Claire McCaskill in Missouri, who had shown appeal to these voters earlier in their careers, each carried only about on -third of them while losing their Senate seats to Republican challengers. In southern states, where many non-college whites are also evangelical Christians, the numbers were even more lopsided: Ron DeSantis carried almost two thirds of them against African-American Democrat Andrew Gillum in the Florida governor's race; Ted Cruz carried nearly three fourths of them in his Texas Senate race against Beto O'Rourke; and Brian Kemp won over four fifths of them against African-American Democrat Stacey Abrams in their Georgia governor's contest.

Democrats, by contrast, carried 53% of whites with a college education in the national House exit polls. Again, it's difficult to precisely compare that with earlier results because of the changes in the poll's methodology. But on Tuesday Democrats won these voters in 13 Senate races and tied in another (North Dakota). Democrats won about three fifths or more of college-educated white women in Senate races from Michigan, Wisconsin, Ohio, Missouri, West Virginia and Pennsylvania, to Nevada, Florida and Montana. White-collar whites also helped Democrats win governorships in Michigan, Wisconsin, Minnesota and Nevada.

The persistence of these demographic trends through all corners of the country largely explains the lopsided geography in results for the House of Representatives. Republicans suffered very few dents in their dominance of small town and rural House seats last week. Democrats won only three districts whose rural population ranks in the top 20% of the House, while losing two of their own in those ranks, according to tabulations by CNN producer Aaron Kessler.

But the palpable recoil from Trump among white-collar voters in all regions explains why the GOP losses in suburban House seats extended so widely. It was perhaps not a surprise that Democrats ousted many of the last House Republicans who had survived for years in suburbs of otherwise blue-trending metro areas, such as Philadelphia, Chicago, Miami, Denver, Los Angeles and Seattle. But the breadth of the movement toward Democrats among well-educated white voters explains why the GOP also lost suburban House seats last week in Atlanta, Charleston, Houston, Dallas, Kansas City, Des Moines, Oklahoma City, Orange County (CA), and possibly Salt Lake City, all places that had earlier resisted the white collar Democratic tide. Of the 33 Republican-held seats that CNN has called for the Democrats, 26 (or nearly four-fifths) are in districts where the share of college graduates exceeds the national average, according to Kessler's calculations.

These are the new swing states

The result is a politics that is converging across different regions. Political analysts, with justification, are accustomed to thinking of Pennsylvania as a quintessential swing state and Texas as the foundation of the Republican coalition. But the results in both places on Tuesday shared many similarities. In both states, Republicans continued to run very well in small towns and rural places (though Democrat Bob Casey in Pennsylvania regained ground in such mid-sized blue-collar communities as Erie and Scranton). And just like Casey, Beto O'Rourke ran up the score in diverse and younger urban places (he won each of the counties including Austin, Dallas and Houston by 200,000 votes or more and amassed a 100,000-plus margin in San Antonio), and made marked gains in previously red-leaning suburban areas around those metropolitan centers, from Collin and Denton counties near Dallas to Williamson and Hayes outside of Austin. Until now, Republicans had dominated both the urban and rural parts of Texas since the mid-1990s. In terms of its political alignment -- a suddenly competitive contest between growing Democratic strength in metro areas and undiminished Republican dominance beyond them -- 2018 may be the year that Texas rejoined the union.

Yet for all the consistency of these geographic and demographic trends, the results in the 2020 presidential race may be shaped most by each party's ability to defy them -- or at least to bend them at the margins. In the Rust Belt states that tipped the 2016 election to Trump -- especially Michigan, Wisconsin and Pennsylvania -- Democrats won both Senate and governor's races last week by restoring an old advantage that Trump had largely erased: In all three states, the Democratic nominees ran slightly better among whites without a college degree than their party did almost anywhere else. They'll need to continue that revival to recapture the states in two years from Trump, who may present a more formidable challenge in them than the Republicans on last week's ballot.

Conversely, despite the Democratic gains in Southern suburbs, Republicans continued to run better among college-educated white voters in Florida, Texas and Georgia than they did in most places. (Although exit polls weren't available in North Carolina, the voting results there point to a similar conclusion.) To hold those states in 2020, Trump will need to prevent any further erosion, even as white-collar voters elsewhere appear to be hardening in their hostility to him.

Even in a politics increasingly homogenized by the same national trends, these small regional variations could provide the tipping point for the epic confrontation approaching in 2020.