Democrat Kim Schrier won the competitive 8th Congressional District over Republican Dino Rossi, capturing 52.9 percent of the vote after a second round of returns were released Wednesday.

KING 5 called the race based on trend lines and our analysis of the vote left to be counted.

Schrier (53 percent) widened her lead over Rossi (47 percent) to 14,949 votes as of Thursday. Her biggest lead is in King County, which is the most populous portion of the 8th District; 57.9 percent of the county voted for Schrier.

Schrier said Wednesday it felt great to be the first Democrat to hold the 8th District since it was created.

RELATED: Track election results

“This is incredible,” said Schrier. “We've never had a Democrat in this seat. We also don’t have any women doctors in Congress until right now. So, we're making history in a couple ways – one by just putting a Democrat in this particular district, but now we have a woman doctor in the House, and that's a really critical missing voice. So, I'm honored, and I will carry this torch and be the best possible representative I can be for the people of this district."

On the county breakdown, she added there were some surprises in the diverse district. Schrier said she knew she would perform well in King County, but did better than expected in Pierce County and east of the Cascades.

She believes her message on health care found support there.

“I look forward to proving myself to be somebody who works with everybody and for everybody just like I’ve done in my office,” Schrier said.

Schrier’s staff added Wednesday that Dino Rossi had not called to concede.

Rossi released a statement Wednesday night, that said in part:

"While this race did not end in the way you or I would have liked, I urge you to stay involved in the democratic process. We all need to stay informed, get involved, and hold public officials accountable for the decisions they make. Remember, the next election is just two short years away. Our country is strongest when people get involved in their communities, whether in political or non-political ways."

Read Rossi's full statement here

The 8th District was an open seat this year since longtime Congressman Dave Reichert is retiring. A Republican has held the position since it was created in 1980, but the district remains a prime target of Democrats looking to pick up seats and take back control of the U.S. House.

The district itself is vast and diverse, spanning 7,000 square miles and four counties, including parts of Chelan, Kittitas, King, and Pierce counties. It stretches from the eastern suburbs of Seattle across the Cascade Mountains into rural agricultural communities.

RELATED: Kim Schrier, Dino Rossi clash on economy, health care in Congressional debate

It's also among nearly two dozen districts in the country currently held by Republicans but whose voters also chose Hillary Clinton over Donald Trump in 2016.

WATCH: Dino Rossi speaks on election night

One of the costliest races in the country, the 8th District race surpassed $28 million in spending, nearly $19 million of which is from outside groups, according to the Center for Responsive Politics. A good portion of that money went toward negative attack ads, which flooded the 8th District in the weeks leading up to Election Day.

Schrier, a pediatrician from Issaquah, entered the race last August. She said she was compelled to run after the GOP-led effort to repeal the Affordable Care Act last year.

FACT CHECK: Attack ad targets Dr. Kim Schrier's medical practice

She raised over $6.9 million, according to the Federal Election Commission. Schrier has also attracted the support of national groups, including EMILY’S List, which supports female candidates.

WATCH: Schrier speaks on election night

Rossi, who was elected to the State Senate in 1996, has spent the last two decades in Washington state politics. He’s perhaps best known for his statewide races, beginning with his 2004 gubernatorial bid against former Governor Christine Gregoire. That race drew national attention as one of the closest races in U.S. history and ended with a trial and two recounts.

FACT CHECK: Attack ad targets Rossi's real estate career

Rossi challenged Gregoire again four years later and ran against Sen. Patty Murray in 2010. In his race against Murray, Rossi lost the statewide vote but won the 8th Congressional District.

He was most recently appointed last year to fill the State Senate seat of late State Sen. Andy Hill.

Rossi raised over $4.4 million.