On a sticky summer weeknight, parents sat on lawn chairs in the grass. Children rode a zip-line and drew in chalk on the asphalt. A rock band played ’90s hits from R.E.M. and Pearl Jam. A food truck sold mango lassi.

The November election was months away, but Rep. Barbara Comstock (R) was handing out balloons and meeting constituents. Democrats supporting her challenger, state Sen. Jennifer T. Wexton, set up a tent and chatted with voters.

Beneath the idyll of suburbs like Brambleton, a planned community less than 20 years old that lies about 30 miles west of Washington, a fierce battle for Virginia’s 10th District and control of Congress is underway.

Brambleton sits in Loudoun County, in the middle of the 10th District, which has been in Republican hands for more than three decades.

But as more Asians, Latinos and college-educated younger people have moved into the congressional district in the past decade, the Republican grip has slipped. In 2016, Hillary Clinton carried the district by about 10 points; in last year’s gubernatorial contest, Democrat Ralph Northam beat Republican Ed Gillespie there by 12 points.

Now, Democrats say the demographic changes, combined with President Trump’s unpopularity — especially in voter-rich Loudoun County — have finally put the congressional seat within reach.

Much of the sprawling 10th District is decided territory — Comstock is likely to win die-hard Republicans in the rural Shenandoah Valley, while Wexton is expected to do well in liberal Fairfax precincts.

But Brambleton is home to the type of diverse, well-educated voters, many of them new arrivals, who will decide the race.

Comstock and Wexton are in a struggle to win these voters, and their strategies reflect the challenge each party faces across the country in November.

Just like Democrats in battleground races throughout the country, Wexton is working hard to nationalize the race and capi­tal­ize on Trump’s unpopularity to convince voters — particularly educated suburban women — that they need to elect Democrats as a check on the president.

Comstock and other Republicans in swing districts have the opposite goal — make the race about local issues, emphasize an ability to deliver for constituents and aim to create some distance between themselves and Trump.

'The tipping point'

When Barack Obama was elected president in 2008, the Loudoun County Democratic Committee could count on only about three dozen people to turn up at meetings. But since Trump’s election, three times as many routinely pack the get-togethers, said the party’s county chairman, Alfonso Nevarez.

The party once struggled to field a congressional candidate. This year, more than a dozen expressed serious interest and six ultimately competed for the nomination.

The district’s proximity to Washington — a slice of it lies inside the Beltway — means national politics are inescapable. And Trump’s election inspired a degree of progressive fervor and engagement not seen there before.

“Trump was the tipping point for me,” said Emily Ticknor, a 33-year-old stay-at-home mother. She and her husband, who works in cybersecurity, moved to Brambleton three years ago in search of a family-oriented community.

Though an active voter, she said she didn’t grasp the consequences of Trump’s election after giving birth to her second child. She said she went out in the rain with her 3-week-old son and toddler daughter to buy diapers and a car almost ran them over.

The driver rolled down her window and yelled, “Why don’t you go back to where you came from?” she said. That’s when it hit her: “This is really happening.”

Ticknor, whose biological parents are Cuban, was born and raised in the United States and speaks only English. She and her four siblings were adopted by white Southern Baptists. Growing up in South Florida, she never encountered overt racism, she said.

“Trump brought out an underlying hatred in Americans, and he’s emboldened them to say and do whatever they want,” she said. “We’ve become a country of bullies.”

She will not vote for Comstock, because the congresswoman is in lockstep with Trump’s Republican Party, Ticknor said.

From January 2017 through the end of July 2018, Comstock voted with the president’s agenda 98 percent of the time, according to the website FiveThirtyEight.

“My husband jokes, it could be a bologna sandwich — if it’s a Democrat he’s going to vote for it,” she said.

For Ticknor’s Brambleton neighbor Amanda Dowling, the 2017 school shooting in Parkland, Fla., changed everything.

“Voting was not a priority for me,” said Dowling, who moved to Brambleton five years ago from Arlington. “It wasn’t something that I took seriously until the past four years or so. While I did vote from time to time, I was not the most educated voter.”

Dowling, who has three children under 5 years old, was struck by the similarities between Brambleton and Parkland — both safe, suburban communities with good schools. Since the shooting, she said, she thinks about how she would feel if something as tragic happened in her town.

“I don’t want to have any regrets,” she said. “I want to do something now and hopefully make a difference.”

She had never attended a rally on the Mall until the March for Our Lives in the spring to protest gun violence.

She joined the local chapter of Moms Demand Action and learned that as a state lawmaker in 2012, Comstock voted to repeal the state’s one-per-month limit on handgun purchases.

“When I heard that, it just sort of left a bad taste in my mouth,” Dowling said. “That’s not the person I want to represent us.”

The two Loudouns

Loudoun County is one of the wealthiest enclaves in the United States and the third-most-populated county in Virginia.

Route 15, a north-south highway, is an informal dividing line. To the west are farms and wineries, and residents who want to maintain their semirural way of life. They once considered seceding from Loudoun.

To the east, developers have built not just subdivisions but whole towns, such as Brambleton, to accommodate hundreds of thousands of newcomers who have moved in since the turn of the millennium. Today nearly 400,000 people live in Loudoun, up from about 170,000 in 2000.

But the changes in the county — to its character, culture and politics — have been about more than sheer density.

The newcomers differ from many longtime residents in key ways. Loudoun’s population of foreign-born residents has more than doubled to 24 percent. More Asians and Hispanics have moved in. And more college-educated newcomers, many of them young and people of color, are calling Loudoun home.

Those demographic changes favor Democrats and, combined with the Trump factor, give Wexton an edge, analysts say.

“The 10th might be gone for Republicans at this point,” said David Wasserman, who tracks House races for the Cook Political Report.

Avram Fechter, vice chairman of the Blue Ridge Democrats, put it this way: “Every new development that goes into Loudoun County turns the county bluer.”

Still, when turnout is low, such as in local and midterm elections, Republicans tend to win. That’s one reason the county board of supervisors remains under GOP control, despite the demographic shifts.

The challenge this year for Democrats is to turn out voters for a midterm as if the presidency were on the line.

Local bona fides

To combat the onslaught of new and newly energized voters, Comstock says she is doing what she has always done. Republicans and many Democrats say she’s an exceptionally hard worker. She maintains a relentless schedule of festivals, parades and picnics in hopes of convincing voters one at a time that she is her own woman.

That strategy paid off for Comstock in 2016. While Clinton carried the 10th District by 10 points, Comstock won reelection to her second term by six points.

If Comstock can persuade those voters to stick with her this year, she could mitigate the damage in Loudoun and win a third term.

Ram Venkatachalam, 37, is a consultant who helps businesses with technology management. He grew up in India, came to the United States to attend college and moved to Brambleton 10 years ago, part of the wave of Asian newcomers. He is running for school board and considers himself a fiscal conservative.

Venkatachalam did not vote in 2016, because he didn’t like Clinton or Trump.

But he is voting in November for Comstock, and he walked her around at the Brambleton concert night, introducing her to his neighbors.

He said he was impressed by her attention to the opioid crisis and her support for Trump’s tax cuts. He likes that she is a frequent visitor to local mosques such as the Adams Center in Sterling and that she opposed Trump’s travel ban affecting visitors from several Muslim-majority countries.

It is her focus on local issues that makes John Bowman a Comstock voter as well. Bowman, 44, grew up a Democrat in Baltimore but became a Republican after the Sept. 11 terrorist attacks to support President George W. Bush. He has voted for Republicans since then, including Trump.

“I think he says some dumb stuff sometimes, but good Lord, everybody’s got to chill out,” he said. “If he’s successful, we’re all successful.”

Bowman considers himself a “middle-of-the-road guy” and said he will vote for Comstock on the strength of her constituent services.

He said her Facebook messages, emails and newsletters show she is looking out for him when it comes to issues that affect his daily life, from transportation to schools.

“The number one thing I would tell you about Barbara Comstock, she does a very good job at communicating in the community,” he said. “I’ve seen her perform in this community, and I support her.”

She needs many more John Bowmans to withstand a potential blue wave.

The rub is that Comstock could do it all — epic fundraising, meticulous constituent services and an effective message — and still lose.

Former state delegate Thomas A. “Tag” Greason, a Republican, found out the hard way that engaging the community may not be enough in the Trump era.

For eight years, he represented parts of Loudoun, including the northern end of Brambleton, and liked to think voters judged him as an individual.

“It was my proudest moment when someone would say, ‘I’m a lifelong Democrat, but I vote for you because of what you do in the community,’ ” Greason said.

Then Clinton won his district by double digits in 2016 and by the time he faced reelection the following year, everything had changed. He lost by about 17 points.

“They came in droves, and they didn’t care if I went to their school or their swim meet or knocked on their door,” he said, referring to voters who cast ballots for his opponent, Democrat David Reid. “There’s still this very, very visceral reaction two years later to the president.”

Greason said there is no question that he will vote for Comstock, and he immediately launched into a vigorous defense of her campaign prowess.

But asked whether he thought she would win, he started to speak and then caught himself, considering his defeat last year, the fundamentals of this particular race and the current political climate. He said he couldn’t offer a prediction.