Over the last four years, Texas has added more voters in the 22 counties along Interstate 35 than in the state’s 232 other counties combined.

Since 2016, Texas voter rolls have grown by almost 2 million voters. More than 1 million of those voters live in communities along the I-35 corridor, sometimes likened to a “blue spine.”

That slice of Texas geography — extending northeast from the border through San Antonio and Austin and all the way to Dallas — has become steadily less conservative and more Democratic. It has helped make Democratic candidates more competitive in statewide contests over the last two election cycles.

In 2018, Democrats flipped 12 seats in the Legislature in districts along I-35, and a strong Democratic showing along the corridor helped Beto O’Rourke come within 2.6 percentage points of defeating incumbent Republican Ted Cruz in the U.S. Senate race. It would have been the Democrats’ first victory in a statewide election in Texas since the 1990s.

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At the same time, voter registration numbers have dropped or remained flat in Republican-leaning counties in the Panhandle, South Texas and East Texas.

“Texas Democrats are rising and showing our strength from South to North Texas,” said Cliff Walker, deputy executive director of the Texas Democratic Party.

Growing voter registrations 10 Texas counties with the biggest vote registration gains since 2016 1. Hays, 31% 2. Williamson, 26% 3. Comal, 24% 4. Brazos, 24% 5. Travis, 23% 6. Rockwall, 23% 7. Denton, 22% 8. Parker, 21% 9. Collin, 21% 10. Guadalupe, 20% Primary 101 Voting Early voting begins Feb. 18. To cast ballots in person, registered voters must bring an acceptable form of photo identification. Those are: a Texas driver’s license, a Texas Election Identification Certificate issued by the Department of Public Safety, a Texas personal identification card, a Texas handgun license, a U.S. military identification card, a U.S. citizenship certificate containing a photo or a U.S. passport. Voters can bring printed materials, such as a sample ballot, into the booth, but may not use cellphones or photographic devices while casting ballots. Voters Guide Everything you need to know about this year’s election, only on HoustonChronicle.com and ExpressNews.com.

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Walker said the I-35 corridor is loaded with rapidly diversifying suburbs and college-educated adults who have gravitated to the Democrats over the last four years.

But none of that means Texas is about to turn blue, said longtime GOP strategist Karl Rove.

“They don’t have a shot as long as Republicans use this as an opportunity to get our act together,” Rove said.

Republicans have to be more aggressive about registering voters and mobilizing volunteers, he added.

The influx of new residents combined with the Democrats’ more aggressive voter registration efforts have allowed the party to find more voters that side with them in elections, said University of Houston political science professor Brandon Rottinghaus.

“The problem Democrats had for the longest time was that they didn’t have enough people to even talk to,” Rottinghaus said.

But now, he said, as the counties along I-35 change, Democrats are able to identify voters who better align with their issues.

Texas is one of a few states in which voters do not register with a political party when they sign up to vote. But demographic characteristics and recent voting trends show that voters along I-35 have shifted from voting mostly Republican to mostly Democratic over the last 4 years.

An Austin-area surge

The biggest surge of new voters has come in two counties on either end of Austin.

Williamson County, just north of Austin, has seen a 26-percent increase in registered voters since 2016.

Just south of Austin, Hays County experienced a 31-percent increase.

O’Rourke carried both counties in the 2018 Senate race.

After Williamson and Hays, the biggest gains in registration were in Comal, Brazos and Travis counties. All three have seen increases of more than 23 percent since 2016.

In Bexar County, the number of registered voters increased 16 percent, making it the 26th fastest-growing county in Texas for voter registration.

Harris County — the state’s most populous — saw a 14-percent increase.

By contrast, in 82 Republican-leaning counties, mostly in the Panhandle, West Texas and East Texas, registered voter rolls have grown 2 percent or less over the last four years. In 55 of those counties, the numbers have declined — by a total of more than 7,000 voters.

(The latest registration totals from the Division of Elections don’t include thousands of new voters likely to have registered since January. Updated figures are expected in about two weeks.)

GOP responds

Republicans have answered Democratic registration efforts with their own drive to sign up voters.

A super PAC created last year called Engage Texas reported that it has raised more than $11 million for a registration campaign.

A second group organized by the Republican Party of Texas, called the Volunteer Engagement Project, has promised to add 100,000 more voters to the rolls this year, with help from Rove.

The 22 counties along I-35 are a big reason Democrats have been gaining ground in Texas elections.

In 2014, U.S. Sen. John Cornyn, a Republican, won those 22 counties by more than 350,000 votes. But in 2016, Democrat Hillary Clinton won the corridor by more than 115,000 votes en route to posting the best statewide showing for a presidential candidate in Texas in more than 20 years.

In 2018, Democrats made further gains in the corridor. U.S. Sen. Ted Cruz lost those counties by 440,000 votes in narrowly defeating O’Rourke. That nearly 800,000-vote swing along the “blue spine“ helped make the race closer than any U.S. Senate race in more than 20 years in Texas.

Cruz was able to overcome the shift along I-35 thanks to three key areas of Texas. Cruz targeted the Texas Panhandle, a half-dozen counties in East Texas and the Houston suburbs as he closed his campaign. And those conservative voters delivered, providing Cruz with almost 250,000 more votes than O’Rourke — slightly more than Cruz’s margin of victory statewide.

The I-35 corridor’s clout was felt beyond the Cruz-O’Rourke contest. Five Republican candidates for Congress in those 22 counties, almost all of them heavy favorites, won with less than 51 percent of the vote.

In the Texas House, Democrats flipped 12 seats previously held by Republicans, 10 of them in districts along I-35.

In the Texas Senate, Democrats flipped two seats, both along I-35.

However, the shift along I-35 has not changed the fact that every statewide elected official in Texas is a Republican. The margins grew tighter in 2018, as Republicans Lt. Gov. Dan Patrick, Attorney General Ken Paxton and Agriculture Commissioner Sid Miller all won 51 percent of the vote or less. Four years earlier, all three of those Republicans won 58 percent or more.

Double-digit gains

Overall, Texas has been adding voters faster than it has been adding people.

Since 2017, the state’s population has grown about 5 percent while the number of registered voters has risen 8 percent. The increase has been dramatic in urban areas such as Harris County. The county’s population has grown 4 percent since 2014, while voter registration has jumped 14 percent.

In Bexar County the population has grown 11 percent since 2014, while voter registration has increased 19 percent.

Early voting in the Texas primary starts on Feb. 18 and runs through Feb. 28. Election Day is March 3.