Sanchez, who appeared at the Anaheim Brewery for her election party, was upbeat despite trailing Harris by double digits.
"It's been an exciting campaign and we are getting ready for round two, so we need all of you to be with us," she said.
None of the race's Republican candidates attracted much attention or excitement leading up the primary, although a last-minute independent spending campaign in support of former California Republican Chair Duf Sundheim did apparently help boost his numbers on Election Day.
Still, the race has always seemed like Harris' to lose.
Harris, 51, announced her candidacy within days of Boxer's retirement announcement last year, and she wasted no time locking up high-profile endorsements and donors. She has raised three times as much money as Sanchez and has led in polls throughout the campaign.
Harris is a former prosecutor who served as San Francisco district attorney from 2004 to 2011. She overcame a rough campaign against then-Los Angeles District Attorney Steve Cooley to win the attorney general post in 2010. She has focused on a wide array of issues, including fighting truancy and helping homeowners hurt during the mortgage meltdown.
Sanchez, 56, was elected to Congress in 1996 after she beat out Orange County Republican Bob Dornan in a tough race. For the last two decades, she has represented the cities of Anaheim, Santa Ana, Orange and Garden Grove.
Sanchez serves on the House Armed Services Committee and Homeland Security Committee and has made fighting against sexual assault in the military a signature issue. Her sister, Linda Sanchez, is also a Southern California congresswoman.
While the women were favored for the two top spots, Loyola law professor Jessica Levinson noted that, as recently as last week, more than a quarter of likely voters told the Field Poll they were still undecided. But none of the Republicans were polling well, she said.
"The Republicans really did not bring anyone who garnered any sort of attention. None of the Republican candidates got any traction," she said. "In part it's just demographic numbers. Democrats are 45 percent of the electorate and Republicans are less than a third, and they are losing market share."
This story is part of California Counts, a collaboration of KPBS, KPCC, KQED and Capital Public Radio to report on the 2016 election. The coverage focuses on major issues and solicits diverse voices on what's important to the future of California.
Read more in this series and let us know your thoughts on Twitter using the hashtag #CACounts.