While the result does not yet spell doom for Feinstein statewide, it marks the first time she has not received the party's backing since she first won a Senate campaign in 1994, according to The Associated Press. It shows some momentum with de Leon, who has cast himself as a fresh, progressive candidate. He has criticized Feinstein for not taking strong enough opposition to Trump.
Feinstein and de Leon will first face off in a primary in June. Because of the way the state's elections work, the pair will likely also be on the ballot in the general election in November.
Arizona special election primary: On Tuesday, a special primary election will take place for Arizona's 8th congressional district. Republican Rep. Trent Franks stepped down in December.
At least 10 Republicans and two Democrats aim to win their party's nomination for the April 24 special election. At this point, the GOP candidate who emerges may prove more important. The handicapper Cook Political Report rates the seat as "solid" Republican.
The leading GOP candidates include state Sens. Debbie Lesko and Steve Montenegro, as well as Phil Lovas, a former state House member. A poll released last month showed Lesko and Montenegro tied at the front of pack with 21 percent of support.
Montenegro's campaign was recently roiled by his admission that he received a nude photo from a state legislative staffer. The state senator denies any wrongdoing.
Pennsylvania special election: A contested special election for Pennsylvania's 18th congressional district will take place on March 13. Rep. Tim Murphy, R-Pa., resigned from Congress last year.
The race pits GOP state Rep. Rick Saccone, 60, against 33-year-old Democrat and former Marine Conor Lamb. The western Pennsylvania district has proven reliably Republican, but recent strong performances in special elections at the federal and state level have made Democrats optimistic.
A Gravis poll released this month found Saccone with a 6 point edge, while a Monmouth poll showed a 3 point advantage for the Republican.
The candidates will need to file financial disclosures by Thursday. The reports will show how much cash their campaigns have on hand to make their cases ahead of the election's final stretch.