AUSTIN — State Rep. Jon Rosenthal wasn’t supposed to win his Texas House seat last year. He was too much of a Democrat for the swath of northwest Harris County that had long elected Republicans.

But in the 2018 election, amid buzz over Democratic U.S. Senate candidate Beto O’Rourke and frustration with the Trump administration, the longtime engineer and first-time candidate emerged as one of a dozen Democrats to turn a Republican seat blue.

Now Rosenthal, 56, has a political target on his back. Republican operatives say Rosenthal’s seat is one of about a dozen nestled in the Texas suburbs that they can win back. Most of the hottest races are expected in the Houston area or the Dallas-Fort Worth Metroplex. Nearly $2 million has already been poured into coffers for candidates as both parties brace for the fight.

“The 2020 year is going to be really wild in terms of what outside influences and national parties spend in our areas,” said Rosenthal.

Texas Take: Get political headlines from across the state sent directly to your inbox

Democrats will have to work the hardest to defend their new turf in Harris County, analysts say, after flipping two seats by slim margins in 2018.

In 2020, the stakes will be considerably higher, as the party that controls the House in 2021 will have a commanding influence on redrawing congressional and legislative district maps that will be in use over the next decade, shaping the political direction of the state.

Republicans have set their sights on Rosenthal, who won District 135 by 3 percentage points in his northwest Harris County district, which spans from Jersey Village to Westgate. Further west in Katy, first-time candidate Gina Calanni eked out a win in District 132 against another Republican incumbent by 113 votes.

“We need to take these two seats back to expand the majority and certainly heading into redistricting next session. It’s critical to keeping Texas Republican after the census,” said Paul Simpson, chairman of the Harris County GOP.

Although population growth in those areas is on the rise, Republicans doubt those districts are shifting as liberal as Democrats think. The districts were victim of a “Beto wave,” Simpson said, noting that voters in both 132 and 135 also favored Republican Gov. Greg Abbott.

For subscribers: How Texas Republican lawmakers raised $18M in just two weeks to fend off Democrats in 2020

Democrats are counting on long-awaited demographic changes to widen the margins and keep both Rosenthal and Calanni in office.

“I think the population has changed dramatically over the past few years and I think there’s a lot more anti-Trump sentiment to add fuel to the fire, said Lillie Schechter, chairwoman of the Harris County Democratic Party.

Democrats are staking claims on two other Republican districts in Harris County.

In one of them, the tightest race in the 150-district House came down to 47 votes. Republican Rep. Dwayne Bohac, whose district includes the Addicks Reservoir, prevailed with a margin of 0.1 percent of the vote, making him a top target for Democrats. Voters in District 138 opted for O’Rourke over re-electing U.S. Sen. Ted Cruz and selected a mix of Republicans and Democrats for statewide office.

District 134 is home to the Texas Medical Center and is considered one of the most affluent in the state. Although a majority of voters there have reliably supported nearly every statewide Democrat, Republican Rep. Sarah Davis won there by a comfortable 6 percentage points.

The Democrats are even keeping their eye on races that appear to be statistical long-shots. Republican Rep. Jim Murphy won by 16 percentage points in 2018 in District 133, Hunters Creek Village. East of Friendswood in District 129, Republican Rep. Dennis Paul won by 15 percentage points. O’Rourke lost to Cruz by about 9 percentage points in both those districts, but the Democrats say they are within reach.

“With the suburbs rapidly shifting, we think that these will be toss-up districts in 2020,” said Abhi Rahman, spokesman for the state party. “Trump accelerates that shift, and he’s running at the top of the ticket. Every Republican has to run with Trump at the top.”

‘Beto factor’ fading?

The Republicans say it’s the Democrats who will have more trouble at the top of the ticket, with no O’Rourke.

“I’m not being arrogant when I say this, but our numbers should have been higher according to the polling,” said Rep. Sam Harless, a relatively moderate Republican who won his first election in 2018 by 9.7 percentage points. “The Beto factor was huge.”

“I think the Democrats see a little blood in the water, they’re getting excited, but I think the Republicans will pick back up five to seven seats,” he said.

In total, Democratic and Republican party operatives have identified 34 seats across Texas as potential toss-ups. Of them, 14 were won within a 5-percent margin in the last election. Another 13 contests came within a 10-percentage point margin, and seven are seen as vulnerable for other reasons.

Democrats need to flip up just nine Republican seats to take control of Texas House for the first time in nearly two decades, shifting the power dynamic away from Republicans who run every branch of state government.

Last year’s midterm elections buoyed Democrats’ hopes of regaining the Texas House. Not only did Democrats pick up 12 seats, but O’Rourke carried 76 of 150 Texas House Districts. That makes the party’s strategy relatively simple: win the districts O’Rourke won and the party will take back the House.

Those hopes are perhaps further advanced by the fact that Democrats traditionally perform better in presidential elections, when turnout surges.

Anticipation from both sides has led to heavy fundraising in the Dallas metro area as Republican incumbents try to hold onto their seats, far outpacing fundraising in Houston.

On the defense in Dallas

Many of the districts where O’Rourke surprised Republicans were along the I-35 corridor, forming a blue spine where Texas was once deep red. From the north of the Dallas-Fort Worth suburbs to San Antonio, traditionally Republican districts that hadn’t voted for a statewide Democrat since Ann Richards first ran for governor in 1990 suddenly turned blue.

Although O’Rourke lost the election by 3 percentage points, Democrats claimed a multitude of down-ballot victories, namely House races around Dallas, Houston and Austin. Eight House Republicans held onto their seats by slimmer margins than their party had seen in years.

Political operatives say more than two dozen House seats are flippable in the greater Houston and Dallas areas: 16 in the DFW Metroplex and 11 in the greater Houston area. But candidates in the Dallas area have raised, on average, nearly three times as much as Houston candidates.

Donors gave $1.4 million to candidates facing the biggest battles in the Metroplex, namely Dallas, Tarrant, Collin and Denton Counties. Embattled candidates in Harris, Fort Bend and Brazoria counties raised about $555,000, campaign records show. Twice as many candidates have signed up to run in the Metroplex.

The lowest hanging fruit for Democrats to flip seats is in the Metroplex, said Mark Jones, a political science professor from Rice University. Of the 15 most vulnerable Republicans, nine are in the DFW area, he said.

“Where are Republicans going to be on the offensive the most, it’s going to be in Houston, chiefly trying to knock off Calanni and Rosenthal. Dallas will be the focus on Republican defense efforts,” he said.

Republican incumbents in Dallas-Fort Worth hustled to raise money after this year’s legislative session ended. Sitting Republicans raised $619,000 over two weeks, as a mandatory fundraising blackout was lifted. That amounts to four times as much money as sitting Democrats and Republicans raised combined in the Houston area. They raised $136,000, according to campaign finance reports.

“None of the Houstonians are prepared for a long election haul,” said Brandon Rottinghaus, a political science professor at the University of Houston. “They all have low cash on hand. Maybe they’ll step it up as things get closer.”

Andrea Zelinski reported from Austin; Dylan McGuinness reported from Houston.