WASHINGTON — An influential women's political group is jumping into one of the most closely watched congressional races in the Philadelphia area, giving an early endorsement to a Democrat who hopes to capitalize on President Trump's struggles in moderate territory.

EMILY's List, which played an aggressive role in last year's U.S. Senate race in Pennsylvania, endorsed Chrissy Houlahan on Monday as she builds her campaign against the Sixth District's Republican incumbent, Ryan Costello.

The Chester County-based district is one of a handful of traditionally Republican districts where Trump lost last year, giving Democrats hope that they can use a possible midterm backlash to flip the seat and others like it, and take control of the House. Houlahan, a first-time candidate with military and business experience, has emerged as one of the party's top recruits, and even GOP leaders have said the race could be a telling one for the national picture.

A Democratic win in the traditionally right-leaning district could be a sign of wider Democratic gains, according to Rep. Steve Stivers (R., Ohio), head of the GOP's congressional campaign arm.

"Of our seats, if you want to look at kind of a bellwether, it would probably be Ryan Costello in suburban Pennsylvania," Stivers recently told C-Span. "He's a great young member, a rising star, but he's got a very strong challenger, and I would pay attention to that race … If they win in Pennsylvania against Ryan Costello, they've got a shot to take the  majority."

EMILY's List, which spent more than $5 million helping Democratic Senate candidate Katie McGinty last year, backs Democratic women who support abortion rights. The group noted that if Houlahan wins, she would become the only woman in Pennsylvania's 20-person delegation.

"I'm excited about what [the endorsement] says about the energy here in the  community and what it says about our opportunity to win here in the district," Houlahan, of Devon, said Monday.

Costello's campaign dismissed the endorsement, pointing to McGinty's defeat against Sen. Pat Toomey (R., Pa.).

"This group's involvement in the 2016 PA Senate race proved once again that DC liberal special interest groups can't buy an election in Pennsylvania and the hard working people of PA6 won't let them do it 2018," said an email from Vince Galko, Costello's campaign spokesman. "Congressman Costello remains focused on finding bipartisan solutions to the problems facing his constituents and our nation."

Houlahan, 50, said she was in touch with EMILY's List early on as she considered running her first political campaign. After organizing a bus of 53 women to attend the Washington Women's March in January, in response to Trump's election, Houlahan said she decided to run for Congress.

She touted herself as a leader "who has some good real-world experience, and one who isn't going to be a rubber stamp for the Trump administration."

A former Air Force engineer who worked on missile defense systems, Houlahan has lived in Chester County since the mid-1990s, when she was an executive who helped build the T-shirt and athletic shoe company And1. She has since been a teacher, joining Teach for America and teaching in Philadelphia at age 45, and an executive in nonprofit companies, most recently one that promotes early childhood literacy.

"EMILY's List is thrilled to endorse Chrissy Houlahan in this critical race to take back the House of Representatives," said a statement from the group's president, Stephanie Schriock.

In challenging Costello, Houlahan is running in a district that leans Republican and where Democrats have touted their campaigns many times before, without success. Though Trump lost the district to Hillary Clinton, Costello won it by more than 14 percentage points.

Democrats have spent much of this year painting the second-term representative as a Trump ally and blasting him for every move the president makes. Costello has worked to create a distinct identity.  He voted against the House GOP's health bill and spoke out against the president's decision to withdraw from the Paris climate accord.

Republicans are already firing back at Houlahan. On Monday the National Republican Congressional Committee blasted her for "advancing a broken status quo at the expense of Pennsylvania families," for her opposition to a House bill rolling back Dodd-Frank financial regulations. Houlahan, on Twitter, called the measure "another vote against PA working families."

Houlahan is the only Democrat to formally file to run in the district, though others have suggested they may do so. Berwyn businessman Bob Dettore has said he plans to run, and Mike Parrish, who lost to Costello last year, has also told party leaders he was considering another campaign.