SACRAMENTO, Calif. (AP) — California billionaire Tom Steyer endorsed state Sen. Kevin de Leon on Wednesday in his uphill challenge to U.S. Sen. Dianne Feinstein.

Steyer's backing could bring an influx of cash de Leon badly needs to increase his name recognition. But right now he has no "concrete plans" to form an independent expenditure in support of de Leon, he told the Los Angeles Times .

Steyer is a former hedge fund manager and Democratic mega-donor best known for promoting environmental causes and calling for the impeachment of President Donald Trump.

He's spent at least $20 million on advertisements and signature gathering efforts in support of impeachment. De Leon has said Feinstein, a Democrat, isn't doing enough to stand up to Trump and that he would support impeachment.

In endorsing de Leon, Steyer said the two share "a lot of values."

"I think he's the kind of young progressive that reflects California and would be a very strong advocate for our state nationally," he told the Times.

De Leon faces headwinds in his challenge to Feinstein, who has served California in the U.S. Senate since 1992. Despite serving as leader of the state Senate, the Los Angeles Democrat's name recognition statewide remains very low, and he has limited money to spend on television advertisements to change that.

De Leon's campaign has about $670,000 on hand compared to Feinstein's more than $10 million, according to campaign finance reports released Sunday.

De Leon called Steyer a good friend and a man of incredible integrity.

"Together we will set a new tone in Washington, D.C. to face the threat of climate change head-on, and beat back an Administration that treats our state with hostility," he said in a statement.

Feinstein's campaign did not comment.