Tran got 43 percent of the vote. Provisional ballots are still being counted, and Tran has not conceded.
Sanchez thanked her supporters in Santa Ana Tuesday night. This was the closest race she has faced since first being elected 14 years ago.
In 2008, Sanchez easily won her seat with almost 70 percent of the vote, but public disenchantment with Democratic incumbents over the failing economy coupled with a formidable opponent in Tran, a Vietnamese immigrant and former Assemblyman, has made for a close and, at times, nasty race.
Both candidates have strong followings in the immigrant communities.
Meanwhile, there will not have to be a special election in Mission Viejo. Candidate Bill Barker, who died of an apparent heart attack on Election Day, did not get enough votes to win a seat on the city council.
In county elections, Supervisor Shawn Nelson beat out Anaheim City Council Member Harry Sidhu in the Fourth District. Nelson filled the seat in June after former Supervisor Chris Norby moved to the State Assembly. The county district includes La Habra, Fullerton, Buena Park, Placentia and Anaheim.
Anaheim voters overwhelmingly voted to ban red light cameras in the city. Measure K, which would prohibit red light cameras from being installed at city intersections, passed with a wide margin.
Three challengers for the Capistrano Unified School District Board of Trustees were poised to overtake incumbents, according to unofficial mail-in ballot tallies. John Alpay, Gary Pritchard and Lynn Hatton led against incumbents Ken Lopez-Maddox, Mike Winston and Larry Christensen.
The school district has been plagued by turmoil and parent unrest, fueling recall elections and a feud between dueling reform groups -- the Committee to Reform CUSD, which backed the incumbents, and the Capistrano Unified Children's First group, which backed a group of five challengers.
The Children's First group's Measure H also led with 60 percent of the vote. It would change the way trustees were elected to the board in the school district.
City News Service contributed to this report.