Why AP had to retract its projection that GOP Rep. David Valadao had beaten Democrat TJ Cox

Republican David Valadao was projected by the Associated Press to win his fourth term representing California’s 21st Congressional District at 10:59 Pacific time on Nov. 6, election night.

But as thousands of mail-in and other ballots were counted in the days after the vote, his initial lead of about 4,400 votes over Democrat TJ Cox would soon dwindle. On Nov. 26, the AP and television networks retracted their call when Cox took a 436-vote lead. On Wednesday, Cox claimed victory with a 529-vote margin.

Media outlets look to AP on election night to project winners before all of the ballots are counted. When making a call, the AP examines election day returns, district demographics and absentee and other voting history, among other factors.

In California, an important component of vote history is the percentage of ballots counted after election day, which historically have trended Democratic. In two House races in Orange County and another in the San Joaquin Valley, for example, Democratic candidates who trailed on election night ultimately prevailed by several thousand votes.

In years past, California has counted approximately 25% of its vote after election day, said AP Deputy Managing Editor David Scott, who oversees the news service’s decision desk operations. In 2016, a presidential year, it was closer to 30%. This year, 41% of the state’s ballots were counted after election day.

“In this instance, the vote that came in and was counted after election day, that vote was very different than election day,” Scott said.

The share of outstanding ballots in the Central Valley’s 21st District was close to the statewide figure, with 40.5% counted since election day, he said.

Those included mail-in ballots postmarked by Nov. 6 that arrive within three days and provisional ballots cast by voters whose eligibility isn’t verified until after election day.

The final updates Wednesday from Fresno and Kings counties broke in favor of Cox, helping seal Valadao’s defeat. Kings is the only one of the district’s counties with a Republican voter registration advantage.

Two of the four counties still have votes to count: Tulare County is home to less than 3% of the district’s registered voters, and Kern County has heavily favored Cox in the tallies since Nov. 6. Because both counties contain other congressional districts, the precise number of votes remaining in the 21st is unknown.

Retractions are rare. Scott estimated the AP had to pull back about two dozen of its 7,000 calls this election season, though he said most were due to county vote tabulation errors.

But the race in the 21st District could push the AP to reevaluate its model.

“One of the things we’ll be looking at ... is: Do we need to make changes to how we analyze elections in California with the expectation that more votes will be counted after election day?” Scott said.

maya.sweedler@latimes.com

Twitter: @mayasweedler