California Rep. Adam Schiff has been all over the news this week as the ranking Democrat on the House Intelligence Committee, which is investigating links between Russia and Donald Trump’s presidential campaign.

He started the week by providing the clearest explanation yet of the Russian connection to the Trump campaign and why it should matter to voters. He ended it Friday, by telling The Chronicle’s editorial board it’s hard to have confidence in the Intelligence Committee’s GOP chairman because he has been “providing political cover” to the White House.

“This is no way to conduct an investigation,” said Schiff, as he blasted fellow California Rep. Devin Nunes, R-Tulare, who earlier in the week told Trump that U.S. intelligence services may have picked up “incidental” information on the president and his transition team as part of a court-approved surveillance of foreign powers. Nunes neglected to tell his fellow committee members.

Through it all, Schiff, 56, has maintained his low-key, steady demeanor, one befitting a former federal prosecutor — someone who can methodically dismantle a witness without raising his voice.

“I’ve never seen him lose his cool,” said Peter Dreier, a professor of politics at Occidental College in Los Angeles. “The way he reacted to Nunes this week is about as angry as I’ve seen him.”

Schiff is emerging from relative obscurity to become a leading voice for Democrats in their opposition to the Trump administration. His plain-spoken, 15-minute statement at the beginning of Monday’s Intelligence Committee hearing won raves in resistance corners.

And his elevated profile is raising speculation that Schiff may be well-suited to succeed California Sen. Dianne Feinstein should she decide not to seek re-election next year. Like Feinstein, Schiff is seen as a moderate willing to work in a bipartisan way

Now more people are becoming aware of his straight-ahead manner — even as he excoriated Nunes’ actions.

Schiff criticized Nunes for briefing President Trump on the “incidental” intelligence — information picked up during an unrelated investigation — and said that and other actions compromise Nunes’ ability to lead a bi-partisan investigation of possible Trump campaign connections to Russians.

On Friday, Nunes canceled a public hearing scheduled for Tuesday that was supposed to feature former Director of National Intelligence James Clapper, former CIA Director John Brennan and Sally Yates, who briefly served as acting attorney general in January. Schiff tweeted the cancellation was an “attempt to choke off public info.”

Nunes has been opaque — “inconsistent” in Schiff’s words — about the content of the intelligence on Trump and his transition team. Nunes said it had nothing to do with Russian interference in the November election.

“It’s hard to say anybody can have full confidence after the events of this week,” Schiff told The Chronicle’s editorial board Friday via Skype. “You don’t take information to the White House instead of your own committee if it purports to shed light on one of the things that we’re investigating.”

If the information wasn’t about the Russian investigation and if, according to Nunes “there’s no evidence that anything was unlawfully collected,” Schiff said, “the only thing I can surmise is that this was an effort to give the president some cover.”

To those who have watched Schiff, who was elected to Congress in 2001, evolve as a politician, his measured response this week can be seen as a characteristic that Democrats need at a moment when it would be easy to devolve into partisan cheap shots.

Schiff may not be a glad-hander and ribbon-cutter, Dreier said, but he is the “the smartest politician I’ve ever been around.”

“He’s anticharismatic,” Dreier said. “His charisma comes from being anticharismatic. He represents Hollywood but he’s not theatrical at all. And that’s rare in this media age to have a politician achieve success by being like that.”

Former state Senate President Pro Tem Don Perata, who knew Schiff when both served in the Legislature, said “he’s a progressive but he doesn’t wear it on his sleeve. He doesn’t believe if he talks louder and more that you’ll believe what he has to say.

“Adam would always try to persuade you by his arguments, not appeal to your knee-jerk reaction,” said Perata, who represented the East Bay in Sacramento.

“So much of politics is right place, right time,” Perata said. “And right now, this guy has come up four cherries. If he were to say he’s running (for Senate), I wouldn’t go to Las Vegas and bet against him.”

But a Senate run is a long way off. Until these high-drama moments on the Intelligence Committee, Schiff has been largely unknown outside of his district, which encompasses Glendale, Burbank, Pasadena and parts of Hollywood. It is largely white, 25 percent Latino and well-educated.

Born in Massachusetts — his mother a Republican and his father a Democrat — Schiff’s family moved to Danville when he was 11. He was so quiet that his mother once said he never told her who he was dating. When his classmates at Monte Vista High School voted him most likely to succeed, “I was totally shocked,” he told the Glendale News-Press. “I didn’t think anybody in the school knew who I was.”

He wasn’t even sure he would go into politics. As he neared the end of his undergraduate career at Stanford University, he took both the Medical College Admission Test and the Law School Admission Test. After graduating from Harvard Law School, he went on to become a U.S. attorney in Southern California, and said he never lost a case.

But he lost a lot at politics. The first time he ran for office, he finished 10th out of 14 candidates in a state Assembly race. It took two more runs before he won a seat to the state Senate in 1996. In 2000, he beat conservative Republican Jim Rogan to go to Congress.

While a passion for foreign policy and intelligence work might be anathema to limelight-loving politicians, they are genuine areas of interest for Schiff, say those who know him. He also has long had an interest in transparency and open government, which he said Nunes was ignoring through his actions Friday.

Regardless of Nunes’ behavior, Schiff said that Democrats are not going to walk away from the investigation of a possible Russian connection. Even though the Democratic members don’t have the votes to require the committee to do anything, he hopes public pressure will result in creation of an independent commission to further investigate. So far, both House Speaker Paul Ryan and Nunes have declined to support an independent probe.

“If we were to walk away and say that the chairman is fatally compromised and we’re not going to participate, then none of this is going to get investigated by the House. And that,” Schiff said, “is just unacceptable.”

Joe Garofoli is The San Francisco Chronicle’s senior political writer. Email: jgarofoli@sfchronicle.com Twitter: @joegarofoli