Feinstein told the Washington Examiner on Tuesday that she has known Biden for 20 years and has a longstanding relationship with him.
“I love Kamala, I appreciate her,” Feinstein said, “but that’s what I’m going to do. I feel very loyal to him."
Feinstein said when Biden was chairman of the Senate Judiciary Committee he made her the “first woman” to serve on the panel.
Feinstein first threw her support behind Biden in January before he officially announced his run.
“She’s brand-new here,” Feinstein told the Los Angeles Times about Harris in January when asked if she would support a Harris run for the White House. “It takes a little bit of time to get to know somebody.”
Harris, who previously served as California's attorney general -- and was district attorney of San Francisco, where Feinstein was once the mayor -- was elected to the Senate in 2016.
In 2004, Feinstein said she would not have endorsed Harris for San Francisco district attorney if she had known Harris wouldn’t seek the death penalty for a gang member who killed a police officer, the Examiner reported.
Several other presidential candidates have been endorsed by their state’s senators, including Bernie Sanders, Amy Klobuchar, Elizabeth Warren and Cory Booker.
Biden consistently leads among Democratic presidential contenders in a number of early polls.
Harris' office did not respond to requests for comment, the Examiner reported.