Democrat Mike Levin claimed he was victorious over Republican challenger Diane Harkey in the 49th Congressional District on election night.
The environmental lawyer and champion for clean energy said that retiring congressman Darrell Issa called him to congratulate him and offered help in advice during his transition.
"We have finally flipped California's 49th Congressional District," Levin exclaimed.
The race was one of several contested congressional districts in California that could cause a shift in the balance of power.
District 49, which encompasses Encinitas, Oceanside, Vista, Carlsbad and a small part of Orange County, was represented by Republican U.S. Rep. Darrell Issa who announced his retirement in January.
With 100 percent of San Diego County precincts reported, Levin held a 55 percent to 45 percent lead over his opponent. That doesn't account for votes in Orange County or absentee voters.
The winner has yet to be officially announced.
Republican Diane Harkey earned an endorsement of President Donald Trump who described her as "an extraordinary woman of great accomplishment & potential.
Democrat Mike Levin was praised in a speech by President Barack Obama as a champion for environmental issues and climate change.
The race for the 49th District seat was one of the most closely-watched races in the country.