Whitesides trailed Garcia, R-Santa Clarita, by 973 votes in the Santa Clarita Valley and Antelope Valley district at the conclusion of Friday's vote count, 50.19%-49.81%, then took a 50.4%-49.6% lead Saturday.
Whitesides, a former NASA chief of staff, cautioned on social media, "There are still thousands of votes to count."
There was no comment from Garcia on social media Saturday.
An updated count is scheduled to be released Sunday.
On Friday, Garcia wrote: "It is essential that every vote cast by the people of our district is thoroughly and accurately counted," Garcia said in a statement after the previous ballot count update. "While we await the final results, variations in daily vote totals are to be expected as the process continues."
"I am confident that when all votes are in, our message of economic security, safe neighborhoods and protecting critical programs like Social Security and Medicare will have resonated with CA-27 families."
George Whitesides says Congressman Mike Garcia is 'too extreme' for CA-27 congressional district
In Orange County's 47th Congressional District, Sen. Dave Min, D-Irvine, added 1,635 votes to what had been a 1,078-vote lead over Republican Scott Baugh, to move 2,713 votes ahead of the former assemblyman, 151,254-148,541, 50.5%-49.5%.
Min wrote on social media he is "feeling good about our momentum" in his quest to succeed Rep. Katie Porter, D-Irvine, who vacated the seat to unsuccessfully run for the Senate seat previously held by the late Sen. Dianne Feinstein.
Baugh has not commented on social media since Tuesday.
In Orange County's 45th Congressional District, Rep. Michelle Steel had her lead over Democrat Derek Tran drop to 6,901 votes, 51.3%-48.7%. The Huntington Beach Republican had led by 7,590 votes, 51.5%-48.5%, entering Saturday's count.
In the 49th Congressional District, Rep. Mike Levin, D-Dana Point, increased his lead over Republican Matt Gunderson from 9,748 votes on Friday to 11,157 Saturday, 51.7%-48.3%, 167,787-156,630.
Gunderson is winning in Orange County but Levin has the lead in San Diego County.
Rep. Ken Calvert, R-Corona, added 500 votes to his lead and is 7,880 votes ahead of Democrat Will Rollins in his bid for a 17th term representing Riverside County's 41st Congressional District, 51.4%-48.6%.
It was unclear how many ballots remain to be counted in the congressional races.
There was as an estimated 212,152 ballots to be processed throughout Orange County.