WASHINGTON — The burgeoning fallout from President Trump’s firing of FBI Director James Comey has thrust California’s two senators into a new prominence on the national stage that shows no sign of diminishing anytime soon.

The two Democrats, veteran Dianne Feinstein and freshman Kamala Harris, widely seen as a potential presidential contender in 2020, sit on committees that are at the center of a gathering maelstrom.

Feinstein is the top Democrat on the Judiciary Committee, which oversees law enforcement, and the former chair and ranking member of the Intelligence Committee, where she remains a sitting member. Harris also sits on Intelligence, which is conducting a high-profile investigation of Russian meddling in November’s election. It is running parallel to a separate investigation by the House Intelligence Committee and an FBI probe that Comey had headed.

“This is a time of testing for our institutions and for the people who are responsible for safeguarding them and making sure they work in the interests of the country and not just the party,” said William Galston, a senior fellow at the center-left Brookings Institution think tank in Washington.

“This is a moment of truth for Republican senators,” Galston said, because with their control of the Senate, they must “make sure that, first of all, the conduct of the investigation into possible links between Russia and the Trump administration is not compromised, sidelined or slowed down in any way.”

For California’s two Democratic senators, Galston said, “the right stance ... (is) to be tough but judicious. There’s a way of doing that, and hysterical partisanship will not serve them or the party or the country very well in the long run.”

So far, both Feinstein and Harris have responded in ways that are “emblematic of their different personalities,” said Jessica Levinson, a professor of law at Loyola Law School in Los Angeles. Both are calling for a special counsel to investigate the Russian meddling, but Feinstein “had a more pragmatic, less emotional response,” she said, while Harris appears “very comfortable positioning herself as one of the leaders of the anti-Trump movement.”

Feinstein drew wide media coverage last week for her views on the Comey affair. As a leading voice on national security, her methodical style tends to enhance her reputation as one of the less partisan Democrats.

Harris, a former prosecutor and California attorney general, made her first foray as a senator on national television Thursday to discuss the Comey firing. As a committee member, Harris typically employs a prosecutorial style when questioning witnesses and tends to appeal in her rhetoric to her party’s liberal base.

Trump’s abrupt dismissal of Comey late Tuesday stunned Congress, and in reaction, both senators homed in on the official White House explanation. Written by Deputy Attorney General Rod Rosenstein, it explained the FBI director’s firing as a response to his mishandling of last year’s investigation of Hillary Clinton’s emails.

Feinstein told reporters on Capitol Hill on Thursday that the memo did nothing to allay her concerns. The motive for Comey’s firing is central because of his role leading his agency’s investigation into possible collusion between Russia and Trump’s campaign.

“I could have written it myself, or my press office,” Feinstein told reporters, referring to Rosenstein’s memo. Feinstein amplified her concerns later at a Judiciary Committee hearing, where she said she had read the memo three times, and laid out at length its lack of legal or other substantive analysis.

Feinstein’s comments were soon overtaken in the news by Trump’s boast in a national network television interview that he had wanted to fire Comey long before doing so was recommended by the top two Justice Department officials, and that the Rosenstein memo was irrelevant. Responding to a request by Feinstein and Judiciary Committee chairman Chuck Grassley, R-Iowa, Rosenstein has agreed to brief the full Senate on the matter this week.

Harris was one of just six Democrats who voted against Rosenstein’s confirmation last month, on the grounds that he had refused to commit to appointing, if necessary, a special prosecutor on Russian meddling.

In her CNN interview, Harris sought to poke holes in Trump’s claim, which accompanied the firing, that Comey had told him three times that he was not under investigation.

Highlighting her experience as a prosecutor, Harris said, “it is the practice of law enforcement, especially when we’re conducting an investigation, that we never talk to someone who may be in the zone of our investigation and tell them that they are not someone we are looking at. That just does not happen.”

Harris also sought to put the firing “in context,” comparing the firing of an investigator by a “potential target of that investigation” to the experience of ordinary citizens involved with the criminal justice system.

“There is everything wrong with that,” Harris said. “Every day, people are walking into courthouses around our country. They’re victims, they’re witnesses or they’ve been accused of a crime, and what must they think ... you can just fire the cop and you can fire the prosecutor? That’s wrong.”

By Friday morning, Harris’ interview was also overshadowed. News reports began calling into question Trump’s account of his dinner with Comey, and Trump responded with a series of tweets, including a threat that “James Comey better hope that there are no ‘tapes’ of our conversations before he starts leaking to the press!”

The House Judiciary Committee’s ranking Democrat, John Conyers of Michigan, and House Oversight and Government Reform’s ranking Democrat, Elijah Cummings of Maryland, sent a letter to the White House counsel saying Trump’s actions could constitute criminal obstruction of justice and demanded release of any purported tapes.

Feinstein sent a letter demanding any tapes as well Friday.

“If President Trump actually has recordings of his conversations with FBI Director Comey, that is a very serious matter, particularly since he appears to be threatening the FBI director with their release,” Feinstein said. “Any recordings must be preserved by the White House.”

Both Feinstein and Harris have consistently called for an independent investigation of Russian influence in the election but are limited in what they can do by Republican control of Congress. Their strongest option is to broadcast their views to the public and generate political pressure on Trump’s allies, including Republican senators, to concede to an independent probe.

While several GOP senators have expressed concern about Comey’s firing, none has called for an independent special prosecutor. Republicans have much to weigh, said John McGlennon, a professor of government at the College of William and Mary in Williamsburg, Va.

“The key question Republicans have to ask themselves is: Do they reach a point where it’s worth it for them to withstand the fury of their base because they’re concerned about the broader implications of Russian involvement in the next midterm and presidential elections?” McGlennon said.

Meanwhile, Levinson said, Feinstein and Harris both carry strengths they can employ as the situation unfolds.

Feinstein could argue, Levinson said, that “now more than ever we need experienced leadership,” while Harris can “use her background to talk about how we need special prosecutor and how these actions are unacceptable. Both of them have the experience to say, ‘You need me in this environment.’”

Carolyn Lochhead is The San Francisco Chronicle’s Washington correspondent. Email: clochhead@sfchronicle.com Twitter: @carolynlochhead