Beto O'Rourke exposed a blue spine in Texas politics that could remake the state's congressional delegation and affect President Donald Trump's re-election prospects in 2020.

By making a strong showing in the 21 counties along the Interstate 35 corridor from Laredo to the Oklahoma border, the U.S. Senate candidate from El Paso defied 30 years of political history in the Lone Star State. O'Rourke didn't just become the first Democratic Senate candidate in Texas to win the majority of votes along the corridor since the 1980s. He pounded U.S. Sen. Ted Cruz along the route, flipping counties that had not voted for a Democrat for statewide office since Ann Richards first ran for governor in 1990. Even in the counties O'Rourke lost, his defeats were often much narrower than those of past Democratic candidates.

"This is a major structural problem for the GOP going forward," said Jay Aiyer, a political science professor at Texas Southern University in Houston.

Texas's population growth has been dramatic in the urban and suburban communities along I-35, while areas that the GOP has long relied on in West Texas and East Texas are losing both population and voters. In other words, the Democratic base is expanding significantly, while the GOP's base is growing less or even shrinking, Aiyer said.

Cruz narrowly won re-election by about 219,000 votes, out of more than 8 million cast, a margin of less than 3 percentage points, according to unofficial results. That's the closest U.S. Senate race in Texas in 40 years. Republicans had won the last four Senate races in Texas by 1 million votes or more. Cruz hung on by running up big numbers in the Texas Panhandle, East Texas and the northern Houston suburbs.

For subscribers: How Ted Cruz survived Beto O’Rourke and the blue wave

But while Cruz stuck with a tried-and-true Republican playbook, O'Rourke revealed untapped opportunities for Democrats. Four years ago, U.S. Sen. John Cornyn won the I-35 corridor by nearly 350,000 votes over his Democratic opponent, David Alameel. But O'Rourke carried those same counties by more than 440,000 votes. That is a nearly 800,000-vote swing in just four years.

And the impact of the blue spine went well beyond O'Rourke's race:

Five Republican candidates for Congress in Texas, almost all of them big favorites, won with less than 51 percent of the vote. All five of their districts are along the I-35 corridor, making them all but certain to be targeted by Democrats in 2020.

In the Texas House, Democrats flipped 12 seats previously held by Republicans. Ten of those districts are along I-35.

In the Texas Senate, Democrats flipped two seats, both along I-35. And they nearly took a third seat north of Dallas, where Republican Angela Paxton won just 51 percent of the vote.

Those results were no fluke, says Manny Garcia, deputy executive director of the Texas Democratic Party. He said even in 2016, Democrats could see how suburban and urban cores along I-35 were changing, which prompted the party to be more aggressive in recruiting candidates there, even in districts that were considered solidly Republican.

More Information A shift in demographics, and politics In 2018, the collection of counties along Interstate 35 voted for Democrats more than Republicans for the second straight election cycle. The two demographic trends with the largest impact in that region are increased diversity and a more educated populace. Non-white voters have been the base of the Democratic Party and, since 2016, progressives have found success pulling college educated voters away from the GOP. Amid rapid growth along Interstate 35 from 2010 to 2018, voter registrations went up accordingly and Democrats successfully turned out those voters in the midterm. — Matt Dempsey

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"The fundamentals of Texas are shifting," Garcia said.

What's changing I-35 is what's changing the state, said Aiyer. The state is growing more diverse and more urban. As major cities become more crowded and more expensive, people are moving to surrounding counties for cheaper housing and taking their political views with them, he said.

There is a clear trend line since 2014. That year, Cornyn won the I-35 corridor by almost 350,000 votes. Two years later, Hillary Clinton won it by just over 115,000 votes. This year, O'Rourke won by an even bigger margin: 440,000.

In 2014, 11 of the 16 congressional districts that touch I-35 were held by Republicans, including 10 in which the Republican won 60 percent of the vote or more. This year, only two of those 11 Republicans topped 60 percent.

If Republicans are going to win along I-35 now, they will need to get away from their old strategy of pressing to the right and counting on their most loyal supporters to turn out in large numbers, said George Seay a Republican Party analyst and donor from Dallas. Seay says Republicans need to move toward the political center for broader appeal.

"This is a bugle call to all Republicans to start talking about the issues all Texans care about," Seay said.

Democrat O’Rourke won in the I-35 corridor

The Democratic candidate for Texas senator, Beto O'Rourke, won the highest number of votes in the 22-county trail along I-35, though his opponent Ted Cruz won more counties overall. The map below shows how each candidate fared in this region's fast-growing counties.

Source: Texas Secretary of State | Created by Jordan Rubio

That means more focus on jobs, health care, transportation and education and less on social and cultural issues aimed at core voters, such as same-sex marriage and immigration.

Democrats drawing up new plays

For decades the Democratic playbook in Texas was consistent: win big in the cities, drive up numbers in the Rio Grande Valley and try to minimize losses elsewhere.

But O'Rourke, who refused to hire political consultants, took a different route. For sure, he mined votes in those traditional targets. But he spent a lot more time along I-35 in places such as Waco, San Marcos and Round Rock than past Democratic candidates. Just over the last 12 days of the campaign, O'Rourke made at least 25 stops in counties along the corridor, compared to three for Cruz. And O'Rourke's campaign invested heavily in field staff to get out the vote along I-35.

Chris Evans, O'Rourke's communications staffer, said the campaign didn't start out with that strategy, but it emerged as the candidate and his aides kept returning to places where they saw the most energy, such as Waco in McLennen County and Plano in Collin County.

The campaign created what they called "pop-up" get-out-the-vote offices in Collin, Denton and McLennen counties, something that was unthinkable four years ago when Republicans trounced Democrats in all three. O'Rourke lost all three of those counties, but by dramatically smaller margins in two of them. In Denton County, O'Rourke did 50,000 votes better than Cruz's last opponent. In Collin, he fared almost 70,000 votes better.

Fast-growing Williamson County, just north of Austin, has been Republican turf almost exclusively since 1990 when Richards won the county in her first race for governor. Cruz carried the area by 30,000 votes during his first campaign in 2012. But O'Rourke wiped out that deficit, winning there by nearly 6,000 votes.

How the parties have performed in statewide races in the I-35 corridor

This animated map shows the winning party in each county along I-35 over time. Counties shaded red went Republican while those in blue voted for Democrats.

Source: Texas Secretary of State | Created by Jordan Rubio

What O'Rourke did in San Antonio was equally impressive. Just four years ago, Cornyn carried the county by almost 40,000 votes. This year, O'Rourke won it by 110,000 votes, the most dominating performance for a U.S. Senate candidate in the Alamo City in decades.

Between San Antonio and Austin, Hays County flipped big for O'Rourke. While Cruz and Cornyn both won the county easily by between 6,000 and 7,000 votes in 2012 and 2014, O'Rourke won it by about 12,000 votes.

That surge in Hays County had a lot to do with new voters. No county in Texas saw a bigger jump in new voter registrations from the March primary elections to November. While counties such as Harris and Bexar saw a 2-percent jump in voter registrations, Hays County registrations went up 8 percentage points.

"The demographics here are massively changing," said Don Inbody, who teaches political science at Texas State University in San Marcos.

The same is true in other counties along I-35, where U.S. Census data show a shift toward an older, more diverse, better educated and wealthier electorate since 2010, changes that favor Democrats.

Trouble for Trump?

Down ticket, the blue spine shook up the Congressional delegation. Some districts that haven't been decided by less than 20 points in past cycles suddenly became the scene of tight battles.

Seay said it had to be a startling night for a lot of longtime Republicans who have never seen a competitive race.

"I think the application of smelling salts to a lot of these guys isn't a bad thing," Seay said.

In Williamson and Bell counties north of Austin, U.S. Rep. John Carter, R-Round Rock, had won his 2016 election by 22 percentage points. The Cook Political Report in Washington, D.C., which tracks all 435 House districts in the nation, didn't even list it among 82 potentially competitive seats.

But then Air Force veteran MJ Hegar caught fire with a viral ad and an aggressive campaign that put Carter on the ropes. She quickly became one of the most well-funded Democrats in the state. Carter survived, barely, with 50.6 percent of the vote.

Further north on I-35, U.S. Rep. Kenny Marchant, a Republican from the north Dallas and Fort Worth suburbs faced underfunded Democratic opponent Jan McDowell. She raised just over $100,000 in a district that Marchant had won easily every two years since 2004. But this year, Marchant won just 50.7 percent of the vote to earn a new term.

If the Democrats had a stronger candidate, Dave Wasserman, an analyst for the Cook Political Report, said the party could have picked up another seat in a fast-changing district that he says is now majority non-white and that gave President Trump just 51 percent of the vote in 2016.

If Trump runs for re-election, he'll likely have to pay more attention to Texas' I-35 corridor and the suburbs. Where Cruz struggled, Trump is apt to struggle too, said Inbody.

"The problem the president has is in those counties up and down the corridor," he said. "You get a few miles off the corridor and you get a whole different story."

Inbody said if Texas is at play in a presidential election, that's a big problem for Republicans. Reliably Democratic California and New York have a combined 82 electoral votes. Republicans need the combined 67 electoral votes in Texas and Florida to act as counterweights. If Democrats were to win California, New York and Texas, a presidential election would quickly become an electoral college landslide, even without Florida.

Aiyer said there is no doubt Texas is still a red state, "but it looks like something much more competitive than it was before."

Data editor Matt Dempsey and staff writer Andrea Zelinski contributed to this report.

jeremy.wallace@chron.com