Politico calls this a potential “California quake,” and it would certainly be a stunning development. Barbara Boxer has won four terms for the US Senate in California, but would only be 76 years old when she comes up for re-election in 2016. That’s not an age when most Senators retire — they’re usually just hitting their stride. Her partner from the Golden State, Dianne Feinstein, won her first term the same year as Boxer (1992) in a special election to replace Pete Wilson, and won a new term two years ago at age 79. One might expect retirement talk to center on Feinstein, but instead it falls on the junior partner:

Sources close to Boxer, 74, say the outspoken liberal senator will decide over the holidays whether to seek reelection in 2016 and will announce her plans shortly after the new year. Few of her friends believe she will run for a fifth term. Boxer has stopped raising money and is not taking steps to assemble a campaign. With Republicans taking over the Senate, she is about to relinquish her chairmanship of the Senate Environment and Public Works Committee. If she were to step aside, it would be the first big crack in the state’s upper political ranks in years. The last time the governorship was open was in 2010, when Jerry Brown, now 76, romped in a return to the job he first held more than three decades earlier. Boxer and California’s other senator, Democrat Dianne Feinstein, 81, were elected in 1992.

Why now? Alex Isenstadt reports that a lot of politicians want their shot at the brass ring:

For a backlog of up-and-coming pols, their opportunity may finally be arriving — and it will be very hard to to pass up. “There has been a bottleneck at the top,” said Mitchell Schwartz, a Democratic strategist who was Barack Obama’s California campaign director in 2008. “In a state of 37 million-plus [population] … elected officials either need to move up or they are out of the game and forgotten quickly.”

Yeah, well. Isenstadt calls this a “quake,” and in a promo e-mail from Politico it’s called a “tectonic shift.” At the moment, it would be neither. Let’s say Boxer retired today, and an election was held next month (it wouldn’t be, but bear with me). Who would win? Republicans haven’t won a statewide office in California since Arnold Schwarzenegger, and his first win was a fluke after the Gray Davis recall. His second win came after he all but embraced the Democratic agenda. Tectonic shifts signify actual changes of position, and as Isenstadt reports, that’s unlikely. The next generation of Democrats aiming for Boxer’s seat would be just as dedicated to progressivism and environmental activism, with perhaps a bit more intellectual heft than Boxer usually provides.

Isenstadt notes this himself in a single paragraph midway through his article. Republicans ran Carly Fiorina against Boxer in 2010 with high hopes that the former HP exec would put a dent in Boxer’s support among women. Boxer wasn’t terribly popular at the time among Californians, with various polls putting her in the mid-40s before the election. She still won by 10 points in a Republican wave midterm election.

Fiorina’s name has recently come up as a potential presidential contender for the GOP in 2016, but that news was not met with much enthusiasm. She might reconsider a run for Boxer’s empty seat, though, without the disadvantage that incumbents put on challengers. Despite losing in 2010, Fiorina may be the highest-profile contender that the GOP can run, and she may have more outsider credibility than a candidate from the next-in-line dynamic that will come from the Democrats. It would still be a very large reach, and the open primary system now in place in California doesn’t even guarantee that a Republican could make it onto the general-election ballot.

None of this answers the question of why Boxer is looking for the exits. There have been no indications of serious health issues, although that’s relatively easy to keep quiet. At 76, Boxer would still be younger than many of her colleagues, and she’d probably have a fairly easy ride to re-election. Going back into the minority has to be frustrating, but Democrats have been publicly confident that they can take the Senate back in 2016 — or they were until they saw the scope of the defeat in 2014. Is that the reason that Boxer’s looking for an out now? Or could the Democrats in California be pressing to replace its two Senators with fresh incumbents before voters in the Golden State wise up about the utter disaster that Democrats have created in the state’s pensions and budgets? That’s the real Big One, politically speaking, that will eventually hit California.