The death of pioneering Democratic Sen. Dianne Feinstein has put California Gov. Gavin Newsom under immediate pressure over whom he picks as her replacement, with Democrats clinging to a narrow Senate majority and Congress likely nearing crucial votes over government funding.

Newsom's decision will attract heavy scrutiny, both on Capitol Hill and in California. The Democratic governor previously pledged to name a Black woman to the seat should Feinstein leave before the end of her term but, earlier this month, ruled out appointing any of the candidates seeking the office in next year's election.

Included in that group is a trio of House Democrats: Reps. Katie Porter, Adam Schiff and Barbara Lee, a trailblazer in her own right and the only Black woman running for Feinstein's seat. The senator had already announced she would not be seeking reelection in 2024.

But the timing of Feinstein's death complicates the political calculus for Newsom, who has been working to boost his national profile ahead of a potential presidential bid in 2028.

Lee, a onetime co-chair of the Congressional Progressive Caucus, is popular with liberals and a star among progressives -- as is Porter. Top Black lawmakers, too, have made it clear that they would be upset if Newsom overlooked Lee, no matter his stated reasons, setting up a potential fight with some of the party's most influential figures and powerful constituencies.

Schiff, a former House Intelligence chairman, became popular with many Democrats over his vocal opposition to former President Donald Trump. He has the support of former House Speaker Nancy Pelosi, who remains an influential figure in California politics (especially in the Bay Area, where Newsom began his career). South Carolina Rep. James Clyburn, for years a member of Pelosi's leadership team and a top ally of President Joe Biden, is backing Lee.

"It would be completely unfair to the Democrats that have worked their tail off," Newsom told NBC News in an interview that aired on September 10. "That primary is just a matter of months away. I don't want to tip the balance of that."

Lee and her supporters were infuriated by Newsom's announcement. The congresswoman said in a statement that "the idea that a Black woman should be appointed only as a caretaker to simply check a box is insulting to countless Black women across this country."

"If the Governor intends to keep his promise and appoint a Black woman to the Senate," Lee added, "the people of California deserve the best possible person for that job. Not a token appointment."

Nevada Rep. Steven Horsford, the chair of the Congressional Black Caucus, also rejected Newsom's explanation and insisted Lee be elevated immediately should Feinstein's seat become vacant.

"The CBC stands with many others when we declare that Rep. Barbara Lee remains the most eminently qualified to serve in this role should an opportunity to appoint someone come to the Governor's desk," Horsford said in a statement earlier this month. "His commitment to appoint the best-qualified Black woman in California to serve in the U.S. Senate shouldn't be to a temporary placeholder, but instead, someone who can immediately get to work."

Newsom tried to deflect in the immediate aftermath, telling CNN's Dana Bash last week that the situation was "a hypothetical on top of a hypothetical." In that interview, he touted what he described as his "pretty good record in this space" and promised "to hold myself to account if that hypothetical on top of a hypothetical ever occurs."

Now, though, that hypothetical has become a reality -- and one the ambitious governor is being called on to reckon with speedily. The Senate appointment will be the second by Newsom, who was called on to fill Kamala Harris's seat when she ascended to the vice presidency in 2021. His choice then, California Secretary of State Alex Padilla, a longtime political ally, won a full term in 2022, making him the first Latino from California elected to the US Senate.

There are currently no Black women serving in the upper chamber. Harris and former Sen. Carol Moseley Braun, the first female senator from Illinois and the first Black woman elected to the Senate, are the only two to hold seats in the body. Moseley Braun, like Feinstein, was first elected to the Senate in 1992.

Feinstein's death also leaves Democrats with an even looser grip on their Senate majority. They now outnumber Republicans 50-49, with New Jersey Sen. Bob Menendez under federal indictment and more than half of the Senate Democratic Caucus now calling for his resignation. Were Menendez -- who has insisted he will not leave and could run for reelection next year -- to go, But should he decide to go, Democrats' advantage would hinge on how quickly Newsom and Democratic Gov. Phil Murphy of New Jersey act to fill the vacant seats.

Some of the names floated by various California politicos as potential successors to Feinstein include Secretary of State Shirley Weber, whom Newsom initially appointed to succeed Padilla; Holly Mitchell, a Los Angeles County supervisor; and Angela Glover Blackwell, a civil rights lawyer and founder of the research and advocacy hub PolicyLink. The governor's office has refused to comment publicly on its deliberations and made no mention of the coming decision in its statement mourning Feinstein's death.

Newsom took a little more than a month after the 2020 election before he selected Padilla. But there was no open seat to fill at the time, with Harris not formally resigning from the Senate until two days before the 2021 inauguration.

This story has been updated with additional information.