Longstanding California Rep. Barbara Lee (D) filed paperwork Wednesday to pave the way to join a crowded field to fill outgoing Sen. Dianne Feinstein's (D-CA) seat.

Lee hasn't officially launched her campaign publicly, but an announcement is expected to come by the end of the month, according to a spokesperson. Should she join the contest, she will square off against Reps. Adam Schiff (D-CA) and Katie Porter (D-CA).

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Under California's system, the top two contenders in the primary, regardless of party, head to the general election. This means that the Democratic melee over the seat could spillover into the general election.

Feinstein, one of the oldest senators at 89, officially bowed out of the contest this week, in an announcement that was mired by internal confusion in which the senator appeared unaware that her staff had put out the statement on her exit.

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House Speaker emeritus Nancy Pelosi (D-CA) has endorsed Schiff in the race.