Rich Pedroncelli, Associated Press
Secretary of State Alex Padilla said at least 2 million votes cast in California's presidential primary election have yet to be counted while speaking at a post election forum Thursday, June 9, 2016, in Sacramento, Calif. Although the outstanding ballots are unlikely to swing Hillary Clinton's win in California, down-ballot elections could be affected as counting continues.

SACRAMENTO, Calif. — About 8.6 million people, or 48 percent of California's voters, are estimated to have turned out for the state's presidential primary election this week, according to early projections by state officials.

The figures are considered preliminary because nearly 2.6 million ballots out of the 8.6 million still must be certified, said California Secretary of State Alex Padilla. At a postelection forum on Thursday in Sacramento, Padilla said the final turnout number won't be known for several days.

The 48 percent turnout is higher than California's 31.1 percent turnout in the primary 2012, when President Barack Obama was running uncontested for the Democratic nomination and California Republicans chose Mitt Romney.

But it is lower than the historic 57.7 percent turnout in 2008, when California Democrats chose Hillary Clinton over Obama and Republicans picked John McCain. The 2008 Democratic primary in California was on Feb. 5 of that year, or Super Tuesday. This year, it was at the tail end of the primary calendar.

The respected Field Poll, before Tuesday's election, had forecast turnout this year of 8 million, or 44.7 percent of the state's 17.9 million registered voters.

The higher-than-expected numbers mean "you got more of the people that don't typically vote," said Mindy Romero, director of the California Civic Engagement Project at UC Davis.

Although detailed data is not yet available, Romero said she expected the greater numbers were due to more young people turning out.

Donald Trump secured the Republican nomination a month before California's primary. Also, on Monday, before the Tuesday vote, Clinton reached the number of delegates needed to become the presumptive Democratic nominee, according to a count by The Associated Press.

Padilla said the impact on turnout of the AP report was hard to quantify.

"It's so hard to measure, it's all anecdotal," Padilla said. "It did, it didn't; it motivated some people to turn out with extra vigor, versus it discouraged somebody else."

Romero said she believed the report likely did not have a major effect on turnout because a majority of California votes had been mailed in before the news.

The Field Poll had forecast that as many as 5 million voters would cast mail-in ballots, which had to be mailed by the June 7 primary. California law allows vote-by-mail ballots postmarked by election day and received at clerks' offices within three days.

"We don't know how many ballots were postmarked on or before election day that are in the process of arriving, still in the window to be processed and counted and added to the tally," Padilla said.

Clinton has nearly 2 million California votes to 1.5 million for Bernie Sanders so far. In the Republican primary, Trump has 1.2 million votes, or 75 percent.