President Trump is erratic, lacks stability and hasn’t shown himself fit to hold office, Sen. Dianne Feinstein said Tuesday, but she hasn’t given up the notion of working with him.

“I do not believe he is a good president. I do not believe he is fit to be president. I do not believe that we have the stability in the executive office that we need to have,” Feinstein, D-Calif., said during an interview with The Chronicle’s editorial board.

“I think there’s an erratic behavior there that is deeply concerning,” Feinstein said.

But Feinstein, who is trying to win support for a new assault weapons ban — a bill for which she has no Republican co-sponsors — noted that she would need Trump’s signature on any measure she can get through Congress.

“I don’t have a choice if I want him to sign something that I’m doing like assault weapons,” Feinstein said. “There’s nothing I can really do about it until either a house changes (into Democratic control) or there’s a constitutional crisis.

“So that’s what makes what Bob Mueller’s work so important,” Feinstein said, referring to the special counsel’s investigation into possible collusion between Trump’s 2016 presidential campaign and the Russian government.

Feinstein brushed aside talk of impeachment before Mueller’s probe is finished.

The senator angered many Democrats last year when she said of Trump, “The question is whether he can learn and change. If so, I believe he can be a good president.”

Asked Tuesday whether she still held out hope of working with Trump, she said, “What difference does it make what I hope or don’t hope? I mean, it is what it is. The ability to do something about it is small because we (Democrats) don’t control either one of the houses. And that’s just a fact of life.”

Feinstein said she lost faith in Trump’s ability to be true to his word after his flip-flops concerning two issues that are dear to her: gun control and the fate of young immigrants covered by the Deferred Action for Childhood Arrivals, or DACA, program. Trump canceled the program last year, potentially exposing to deportation about 690,000 young immigrants who came to the U.S. as minors.

But he appeared to be open to giving Democrats an opportunity to propose legislation to protect DACA recipients during a January meeting at the White House that was open to the media.

“We’re going to do DACA,” Trump told Feinstein. But the next day, he reversed field. And this week, Trump tweeted that “DACA is dead.”

Even more painful to Feinstein was when Trump changed his mind on curbing assault weapons. In February, she was among a bipartisan group of 17 legislators who met with Trump for a televised meeting on gun control. There — with the cameras rolling and Feinstein seated next to him — Trump demanded that legislators come together on “one terrific bill” on guns that would expand background checks and perhaps include Feinstein’s proposed ban on assault weapons.

Trump also directed lawmakers to consider Feinstein’s new assault weapons ban. “Dianne, add what you have also,” Trump told her. Cameras caught Feinstein rubbing her hands gleefully at the suggestion.

“And that lasted for 24 hours,” until Trump changed his mind, Feinstein said Tuesday.

“I’ve served under Republican and Democratic presidents,” said Feinstein, who was first elected to the Senate in 1992. “There was always a sense of stability. A president’s word always meant something. It didn’t change the day after.”

Feinstein conceded she has heard from more liberal Democrats that she has not been tough enough on Trump.

“I’m not a name-caller. I don’t call people names. All people want to hear, it appears, are epithets about him,” she said. “My job is to get legislation passed or get problems solved or find money to help solve those problems.”