Congresswoman Barbara Lee’s campaign for Senate in 2024 suffered a blow with Governor Newsom announcing that he would appoint the president of Emily’s List, Laphonza Butler, to fill Senator Feinstein’s seat until the 2024 election.

Mr. Newsom’s announcement came after some of Ms. Lee’s allies, including Congressman Ro Khanna, Congressman Steven Horsford, and Congresswoman Pramilla Jayapal, indicated that they thought their pressure on Mr. Newsom was going to influence his decision.

In a post on X, Mr. Khanna said that “Newsom shifted today” and that Ms. Lee is “the obvious pick for Newsom to be true to his pledge. She is the most qualified and committed to running.”

Ms. Lee had little to say publicly about the appointment Monday, stating only that she is “singularly focused on winning my campaign for Senate.”

“I wish Laphonza Butler well and look forward to working closely with her to deliver for the Golden State,” Ms. Lee said. “CA deserves an experienced Senator who will deliver on progressive priorities.”

Theoretically, Ms. Butler could run for the seat in 2024, with Politico reporting that Mr. Newsom did not put any limitations or preconditions on the appointment.

Mr. Newsom had received pressure to appoint Ms. Lee after promising to appoint a Black woman to the Senate. He later told NBC that he was intent on not affecting the ongoing 2024 Senate race.

Mr. Newsom has already received criticism for his choice. Ms. Butler currently resides in Maryland and will need to register to vote in California in order to be sworn into office. Yet she could assume her seat in the Senate as early as Tuesday.

Without that boost, Ms. Lee will likely remain in third place behind Congressman Adam Schiff and Congresswoman Katy Porter. A September poll from the Berkeley Institute of Governmental Studies found that Mr. Schiff leads the contest with 20 percent support among likely voters, and Ms. Porter is right behind him at 17 percent. Ms. Lee is third with 7 percent.

Other polls have found similar results, with Ms. Lee consistently placing third in the field. The problem for Ms. Lee is that under California’s primary rules, only the top two candidates — Mr. Schiff and Ms. Porter if trends hold — proceed to the general election.

The other issue for Ms. Lee is that both Mr. Schiff and Ms. Porter have raised far more money for their campaigns. As of June 30,  Mr. Schiff had raised nearly $30 million, Ms. Porter had raised nearly $10.5 million, and Ms. Lee had raised $1.4 million, according to Federal Elections Commission data.

Following Ms. Butler’s appointment, California will also need to schedule a special election. Under California law, donations to a special election and a normal election campaign are distinct, meaning individual donors can potentially give an additional maximum campaign donation for the special election, which will likely also take place in 2024.

In real terms, this means that donors can now give a total of $13,200 a candidate, whereas they could previously give a maximum of $6,600 a candidate in the Senate race, $3,300 for the primary, and $3,300 for the general election.

So far, Mr. Schiff has received some 789 maxed-out donations of $3,300. Although some of these will be earmarked for the general election, donors now have the opportunity to effectively double their contributions to Mr. Schiff. For comparison, Ms. Porter has received some 154 maximum donations, and Ms. Lee has received 196.