Tuesday's primary race in the 49th Congressional District was much closer than observers expected.
Douglas Applegate, an attorney and a retired Marine, is posing a stronger than expected challenge for Republican incumbent Darrell Issa.
Even though Issa outspent his opponent by a 10-to-1 margin, Applegate came within a few percentage points.
With 100 percent of the precincts reporting, Issa captured 51 percent of the vote and Applegate garnered 45 percent. Some mail-in and provisional ballots have not yet been counted.
The two will face off in November's general election.
Thad Kousser, a political science professor at UC San Diego, said a couple of things probably hurt the Republican lawmaker.
One was Issa's support for presumptive GOP presidential nominee Donald Trump.
"Being associated with Donald Trump, being in the party of Donald Trump, being an endorser of Donald Trump may have pulled him down, especially in a district that has a lot of Latino voters," Kousser said. "And secondly, with Donald Trump sewing up the Republican race, there wasn't that push to get Republicans to the polls, and that would've helped Congressman Issa."
The other factor was likely the competitive presidential primary on the Democratic side.
"This was a much bluer primary electorate than we've seen in years past in California because this was the center of the world for Democrats. It didn't bring a lot of Republicans to the polls. I think things could look much different in November," Kousser said.
Issa has held his congressional seat since 2000. He has easily turned away challengers in all of his previous elections.
The 49th Congressional District includes Del Mar, Solana Beach, Encinitas, Carlsbad, Oceanside, Vista and San Clemente.