Sen. Dianne Feinstein told reporters Thursday that she would back former Vice President Joe Biden for president if he decides to run in 2020.

“My candidate would be Joe Biden,” Feinstein said. “I’ve seen him operate. I’ve seen him perform and I think he brings a level of experience and seniority, which I think is really important.”

Biden, 76, has talked about challenging President Trump, but hasn’t announced any decision. He was senator for Delaware for 36 years, serving with Feinstein, before becoming Barack Obama’s vice president in 2009.

Feinstein said she has not spoken with Biden about his possible presidential run. The unsolicited endorsement “was just my view,” she said.

The California Democrat’s remarks were first reported by Politico.

Massachusetts Sen. Elizabeth Warren and former Housing and Urban Development Secretary Julian Castro are the only Democrats who have taken official steps toward a presidential run. But more than a dozen others are potentially in the mix — including Feinstein’s fellow California senator, Kamala Harris.

Asked about Feinstein’s remarks, her spokesman Tom Mentzer said in an email, “Senator Feinstein spoke about her many years of working with Joe Biden in the Senate and as vice president. When asked about Senator Harris, she said she very much enjoys working with her and has gotten to know her better over the past two years.”

John Wildermuth is a San Francisco Chronicle staff writer. Email: jwildermuth@sfchronicle.com Twitter: @sfchronicle