James Cargas plopped a folder on his usual table at a downtown Greek restaurant here and explained how it would help him get to Congress. He had tried three times before, every two years since 2012, and the last time, he spent barely $70,000. This time, he said, would be different. Inside the folder, he had stuffed pages of information about the surging Democratic vote in a traditionally Republican suburb.

“You’re here because Hillary won the district,” said Cargas.

In 2016, the unassuming energy lawyer with no money watched Houston’s Harris County go blue for Hillary Clinton. At the same time, Texas’s 7th Congressional District — a wealthy band of neighborhoods enriched by his industry — broke for the Democratic nominee by the thinnest of margins. Four years earlier, it had gone for Mitt Romney over Barack Obama by 21 points.

As Democrats recover from their post-election daze, and as President Trump enacts the policies they ran against, they’ve begun to scour the map to see where and how to take back power. That’s led them to what had been deep-red America, places where Clinton’s long bet on the “emerging majority” of white surbanites and melting-pot nonwhites led to gains.

Clinton’s map included Arizona’s 2nd District, briefly represented by Rep. Gabrielle Giffords (D-Ariz.); Illinois’s 6th District, whose Rep. Peter J. Roskam (R-Ill.) has served in House leadership; and Florida’s 27th District, a Latino-heavy stretch of Miami that voted for Clinton by 20 points while reelecting Rep. Ileana Ros-Lehtinen. Texas’s 7th District is one of at least 22 across the country that voted for both Clinton and a Republican member of Congress; winning 24 would give Democrats the majority.

“All of these districts are really tough, but we’re expanding the battlefield,” said Rep. Ben Ray Luján (D-N.M.), who just began his second term atop the Democratic Congressional Campaign Committee. “President Trump is starting his term with the lowest approval ratings of any president we’ve seen, and that’s after he lost these districts.”

Out-of-power parties rarely pitch a perfect game, and in 2018, a group of Midwestern Democrats will be defending themselves in districts that broke for Trump. On average, presidents polling under 50 percent approval at their midterms have seen their parties lose 35 House seats. Genuinely surprised by the energy of anti-Trump protests, Democrats are already wondering where it could translate into votes.

“You saw it in the Women’s March,” said Rep. Beto O’Rourke (D-Tex.), a congressman from El Paso who has been traveling across Texas as he considers a 2018 Senate bid. “I saw it when I was in Harris County. People who’ve never run for office are trying to find out how. One of my colleagues on the floor actually said to me, ‘Hey, forget Ohio. What happens if we win Texas?’ ”

While Trump defied the usual political gravity in much of the country, in places like James Cargas’s Houston, he sunk. Texas, redistricted by Republicans (after some judicial intervention) in 2011, was drawn so that just one of its 36 districts would be competitive. Based on the results of former President Barack Obama’s two elections, only the 23rd District, sprawling along the Mexican border, was even between the parties. Rep. Will Hurd (R-Tex.), who won the seat in 2014 and 2016, has criticized Trump’s executive order to finish a wall there.

But the Clinton-Trump race shook up the map without shaking up how Republicans campaigned. Clinton won Hurd’s seat, as well as the Dallas-based 32nd District of Rep. Pete Sessions (R-Tex.) and 7th District represented by Rep. John Abney Culberson (R-Tex.). Sessions didn’t even draw an opponent; Cargas, endorsed by the Houston Chronicle, got no national Democratic support.

Both of the incumbents are reliable conservatives who’ve found plenty to like about Trump. Culberson, who was not available to comment for this article, has taken what could have been a low-profile chairmanship of an appropriations subcommittee and turned it into a vehicle for defunding sanctuary cities that do not prioritize deporting undocumented immigrants.

On Friday, on one of his semiregular call-ins to the Boston radio host Howie Carr, Culberson recalled how he passed legislation that would not just deprive sanctuary cities of federal money, but spur them to refund it.

“So the mayor of Boston wants to protect criminal illegal aliens instead of law-abiding Americans?” he said. “Boston’s going to lose that law enforcement money in 24 hours. It’s gonna be quick.”

That’s not an unpopular stance in Texas — indeed, Gov. Greg Abbott (R-Tex.) has threatened to remove public officeholders who defy the sanctuary cities order. But Abbott won in 2014, by a landslide, soft-peddling Trump-like issues and reminding Latino voters that his wife’s family was Mexican. Paul Simpson, the chairman of Harris County’s GOP, said that 2014 provided more keys to the Texas map than 2016.

“It was in many ways, here and nationally, an anomaly of an election,” said Simpson. “A lot of traditional Republicans would not vote for Trump. I’m hopeful that we can bring home a lot of the Republicans who didn’t vote and win over the traditional Democrats who went for Trump.”

Democrats didn’t see the same trendline. In Harris County, the party won local offices that had easily gone Republican in the past. (Simpson pointed out that the Democrats’ successful candidate for district attorney had been backed by George Soros.) The 7th District, which had not gone Democratic since George H.W. Bush was elected in 1966, was part of the story. Fifty-five percent of its voters were black, Latino or Asian American.

“I don’t buy the ‘fluke’ explanation,” said Lane Lewis, chairman of the county Democrats, who is retiring after five years on the job. “Trump was as crazy in the rest of the country as he was in Harris County, and we bucked the trend.”

As chairman, Lewis had set out to expand the Democrat’s mail-in ballot program, starting with the number of Republicans who mailed in ballots and aiming to beat it. In 2016, it worked. At one point, he recalled, the pattern of turned-in ballots suggested that Cargas was at 48 percent support, higher than the party had ever polled there.

What was unclear, 21 months before the next election, was what new Democratic interest in the seat meant for Cargas. In an interview, he said that his 44 percent bested the 42 percent won by Michael Skelly, a wind-energy businessman who raised the most money of any House candidate in America during his 2008 challenge to Culberson.

But national and state Democrats, as they looked for targets, didn’t rule out recruiting someone new. On Saturday, Cargas cornered outgoing DNC chair Donna Brazile at a party meeting in Houston and got an inconclusive answer about why the party had not put more into his campaign.

Meanwhile, at the meeting itself, Democrats were rattling off success stories from 2016 — and saying plenty about Texas. Maria Teresa Kumar, the founder of the turnout group Voto Latino, suggested that Trump was giving Republicans “a Pete Wilson moment,” threatening to activate nonwhite voters as the former California governor had when he cracked down on illegal immigration. Gilberto Hinojosa, chairman of Texas’s Democratic Party, said that the Lucy-and-the-football story of Democrats in Texas was coming to an end, as seen with presidential votes in Harris County.

“In 2012, the margin was 1,000 votes,” said Hinojosa. “In 2016, the margin was 160,000 votes. The margin in 2020 will be 400,000 votes. It’s all over for Republicans in Harris County.”

Republicans, who’ve heard similar promises from Democrats before, weren’t buying it.

“Trump’s comments hurt him and made it a one-off deal,” said Rep. Pete Olson (R-Tex.), whose district in the exurbs of Houston gave a 25-point margin for Mitt Romney and an 8-point margin to Trump. “It won’t happen again. That talk about Texas going blue? That ain’t happening.”