Harley Rouda, Democrat running for California's 48th Congressional district seat in Congress, listens to speakers during his campaign rally in Laguna Beach, Calif., on May 20, 2018.

Democratic candidate and businessman Harley Rouda is the apparent winner in a hotly contested race in California's historically conservative 48th Congressional District, unseating longtime Republican Rep. Dana Rohrabacher, according to NBC News.

Rouda ran a well-financed campaign and saw his support rise in the polls after he and Democrat-backed PAC ads attacked Rohrabacher's record and his pro-Russia views. The 15-term congressman from Orange County previously defended Russia's 2014 annexation of Ukraine's Crimean peninsula and gave Russia a pass on interference in the 2016 presidential election.

"Congressman Rohrabacher has not conceded the race," Rohrabacher campaign spokesman Dale Neugebauer told CNBC in an email late Wednesday. He said there still are "200,000 ballots remaining to be tallied in Orange County."

In the last few weeks of the congressional campaign, Rohrabacher tried to shift attention away from his ties to Russia by launching an ad casting himself as a health-care advocate and fighter of pre-existing medical conditions. Yet the incumbent was called out by the Rouda campaign for having voted more than a dozen times to get rid of protections from pre-existing conditions under the Affordable Care Act.

Rouda, who supports Medicare for all residents, raised nearly $3.2 million in the third quarter, or about 7 times the amount Rohrabacher collected, according to federal election records. Rouda, a real estate developer and attorney, spent about $1.6 million of his own money on the race and received more than $4 million in donations in the final weeks for ad buys from billionaire Michael Bloomberg, the former New York mayor.

Republicans have a nearly 10 percentage point registration advantage in the district, but there have been changing demographics, and Democratic presidential candidate Hillary Clinton won there by nearly 2 points in 2016. The district runs along the Orange County coastline, from Seal Beach to Laguna Niguel.

-Updated: Adds comment from Rep. Dana Rohrabacher campaign.