One San Francisco pol told me he thinks that if Feinstein did get a serious challenger, she would decide not to run rather than have to jump through all the hoops of a serious campaign.

As we saw in last year's race for Barbara Boxer's Senate seat, the top two finishers in the June primary election were both Democrats, leaving Kamala Harris and Loretta Sanchez to face off in November. The chance that two people from the same party could face off changes the calculation of whether or not to jump into the race.

And there's no shortage of Democrats itching to run for higher office.

Several -- including Lt. Gov. Gavin Newsom, state Treasurer John Chiang, former Los Angeles Mayor Antonio Villaraigosa and former state Schools Superintendent Delaine Eastin -- have already announced they're running to succeed termed-out Jerry Brown next year.

But they and others could switch from a crowded governor's race to the U.S. Senate instead if there's an opportunity.

"Her job is to become so formidable that she discourages those folks from taking her on," Gerston said. "But the older she gets, the more people resent incumbency, right or wrong."

'She can hold that seat by safe margins as long as she wants to.' Political analyst Dan Schnur

Feinstein's apparent reluctance to hold town hall-type meetings with constituents and her refusal to debate her last two Republican opponents have contributed to a sense that the San Francisco Democrat is a bit removed from voters.

Still, after 25 years in Washington, Feinstein has tremendous influence as a senior Democrat. She's the ranking member of the Senate Judiciary Committee and she's vice chair of the Intelligence Committee. She also has a deep well of support among party leaders, from Gov. Jerry Brown on down.

"She can hold that seat by safe margins as long as she wants to," said political analyst Dan Schnur from USC. He said it would be a "suicide mission" for another Democrat to take her on.

"Almost anyone willing to take a shot at her is by definition not viable," Schnur said, "and anyone who's viable is going to want to preserve their viability."

In other words, someone who would be a strong candidate for an open Senate seat, like Southern California congressman Adam Schiff, environmental activist Tom Steyer or Los Angeles Mayor Eric Garcetti, wouldn't want to poison their relationship with the party by taking on Feinstein.

And someone willing to take her on, Schnur believes, would likely be someone from the extreme left of the Democratic Party who under California's "top two" primary system probably couldn't win a statewide election against Feinstein in November.

Garcetti, who just won re-election with more than 80 percent of the vote, would seem to have lots of options and potential.

"He has a lot of qualities that would do well in Washington," said Raphael Sonenshein, executive director of the Pat Brown Institute for Public Affairs at Cal State Los Angeles. "He's extremely articulate, telegenic, quick on his feet and has a very good demeanor for television."

Garcetti's landslide re-election this week has surely earned him the right to think about running for governor or the U.S. Senate. But, said Schnur, at age 46 Garcetti can also just bide his time and focus on being a good chief executive of his city. "Being mayor of Los Angeles is not a death sentence or a career capper," he said.

There's also evidence that Feinstein, known for her bipartisan approach in Washington, is getting the message that the activist base of the party is restless and wants more opposition than cooperation with Republicans in Washington. After showing more openness to Trump's Cabinet nominees, Feinstein has taken a harder line in questioning and confirmation votes.

"It shows she doesn't have a tin ear," said Sonenshein.

Feinstein's longtime political consultant, Bill Carrick, believes she "is in good shape" and is "extremely popular."

"We shouldn’t misinterpret the people who are protesting," Carrick said, referring to recent demonstrations outside Feinstein's home in San Francisco. "Nobody’s going to escape that. We're in a moment with a lot of anger. Everybody’s going to get a dose of that."

Still, Feinstein's age and a recent medical procedure to install a pacemaker elevated Democrats' concerns about re-electing someone who will would be 92 at the end of another term.