Gov. Gavin Newsom of California is expected on Monday to name Laphonza Butler, the president of Emily’s List and a former labor leader, to fill the vacancy left in the Senate by the death of Dianne Feinstein, an official familiar with the decision confirmed on Sunday night.

Ms. Butler, a fixture in California politics for nearly 15 years, led the state’s largest union and was an adviser to Vice President Kamala Harris before taking the helm at Emily’s List, a national organization dedicated to helping women win political office.

The appointment is expected to come four days after Senator Feinstein died on Thursday in Washington, D.C. The senator’s body was flown over the weekend back to San Francisco, where a memorial service has been scheduled for Thursday.

This is a developing story. Check back for updates.