Katie Porter, the former Democratic congresswoman who rose to prominence by wielding a whiteboard while she grilled corporate executives on Capitol Hill, announced on Tuesday that she would enter the 2026 contest for California governor.

Ms. Porter, 51, is the highest-profile Democrat to join the race. But a huge unknown remains: whether former Vice President Kamala Harris will jump in.

Gov. Gavin Newsom cannot run for re-election because state law limits governors to two terms. Most of the Democrats who have entered the open race so far are current or former state officials who have experience in the statehouse but are little known to the public. Voters are much more familiar with Ms. Porter, a law professor who served six years in Congress representing Orange County, polls show.

Ms. Porter announced her campaign on Tuesday by releasing a video that cast herself as someone who would fight to counter President Trump’s agenda and bring a fresh lens to the State Capitol, where she has never served in an elected office.

After losing the presidential election in November, Ms. Harris returned home to Los Angeles and has contemplated a run for California governor. Ms. Porter will drop out of the race if Ms. Harris runs, a spokesman confirmed.

Ms. Porter observed in December that Ms. Harris would have “a near field-clearing effect on the Democratic side” of the race.