California Senate President Pro Tem Kevin de Leon and state Attorney General Kamala Harris chat after Gov. Jerry Brown's historic fourth inauguration at the State Capitol in Sacramento on Monday. Lt. Gov. Gavin Newsom is at right. (Max Whittaker/Reuters)

Sen. Barbara Boxer (D-Calif.) announced Thursday that she won't seek reelection in 2016 — a decision that, although not at all surprising, sets in motion a scramble for the future of the California Democratic Party.

Seconds after Boxer's announcement,  talk turned to the prospect of two Democratic heavy-hitters running for the seat: Lt. Gov. Gavin Newsom and state Attorney General Kamala Harris. It's a tantalizing prospect for any political junkie, especially given that the two of them have national profiles and are considered rising stars in the nation's biggest state.

But almost nobody thinks it's going to happen. Here's why.

1) Other opportunities ahead

Sen. Dianne Feinstein (D-Calif.) is the oldest current senator, at 81, and she's up for reelection just two years after Boxer, in 2018. She also just lost her chairmanship of the Senate intelligence committee when Republicans regained the Senate majority (although Democrats could conceivably win it back in 2016).

Also, Gov. Jerry Brown (D) can't run again in 2018, after being sworn in to his fourth (non-consecutive) term this week. He was able to run for four terms because his first two were before the state instituted a maximum of two terms.

Add it up and three major statewide offices could be available in the next four years, after California Democrats were locked out of the state's two Senate seats for 20 years.

And if either Newsom or Harris is more interested in being governor — Newsom briefly ran against Brown in 2010 before opting for LG — the choice could be pretty clear.

2) California's top-two primary system

California is a very blue state, and Republicans have struggled to win statewide office for decades. Even when Republicans had a solid nominee and a good Republican year — Carly Fiorina challenging Boxer in 2010 — they didn't come close. (Fiorina lost by 10 points.)

About the only way Republicans could win is by taking advantage of the relatively new top-two primary system. Basically, a whole bunch of Democrats run alongside two Republicans and split up the vote so much that the general election features two Republicans. It sounds farfetched, but it happened in 2012, allowing then-Rep. Gary Miller (R-Calif.) to hold a heavily Obama congressional district, and it almost happened in the 2014 state controller race.

The more likely outcome, however, is that two Democrats could advance through the open primary system and make it to a general election face-off.  Given the dismal state of the Republican Party in California, that's very much a possibility.

3) Newsom and Harris are friendly with each other

In fact, Harris swore Newsom in this week to his second term as lieutenant governor — a moment interpreted by many as unity between the two, because basically everyone knows they are next in line.

And there's little reason for them to run against each other. They come from the same part of the state (San Francisco) and have very similar political bases — even the same consultant, Averill "Ace" Smith. Competing against each other would only serve to divide those bases and Northern California votes and perhaps open the door to a candidate from the Los Angeles area to shoot the gap.

One senior Democratic operative said that it is unlikely that both Harris and Newsom will run but added that "it's likely one of them will."  The source added, "I think Kamala will have a head start."

California Democratic consultant Jim Ross agrees, at least on the first part.

"The most likely outcome is that one runs for Senate and the other runs for governor" in 2018, he said. "Not that either one is guaranteed a win, but they would be very strong candidates and likely front-runners in their respective races."

Putting aside, Newsom and Harris for a minute, the other big-name candidate who could run (and perhaps be that Los Angeles candidate) is former L.A. mayor Antonio Villaraigosa. Villaraigosa has kept his name in the news in the years since he left office, including serving as chairman of the 2012 Democratic National Convention.

Another name being bandied about is Tom Steyer, an environmentalist who spent $74 million of his own money on the 2014 campaign — an effort widely regarded as unsuccessful. Other Democrats include Los Angeles Mayor Eric Garcetti, San Jose Mayor Sam Liccardo, Rep. Loretta Sanchez, state Insurance Commissioner Dave Jones, state Treasurer John Chiang, former congresswoman Jane Harman and Rep. Karen Bass.

It's hard to handicap the race this far out. A USC-Los Angeles Times poll conducted last year showed that none of these candidates — save perhaps Villaraigosa — are well-known across the state, with Newsom's favorable-unfavorable split at 30-19 and Harris's at 26-12. Half don't even recognize Harris's name, while 58 percent don't recognize Newsom's.

A big reason there's so much focus on these two is that the cost of running statewide in California is prohibitive, and it's very difficult for an unknown candidate who isn't independently wealthy to build a big enough apparatus.

"Just consider that a candidate is going to need to be able to demonstrate the capacity to raise $35 million -- probably closer to $50 million -- just to be competitive for the June open primary," said California Democratic strategist Chris Lehane, "and then raise the same amount in a very short time period for the November general election."

On the GOP side, Republicans have been unable to field strong candidates in recent years. Neel Kashkari fell to Brown by 18 20 points in November, and Fiorina appears more interested in running for president in 2016. Others mentioned include wealthy physicist Charles Munger Jr., 2010 gubernatorial nominee Meg Whitman, San Diego Mayor Kevin Faulconer and 2010 gubernatorial candidate Steve Poizner. Their dream candidate would be former secretary of state Condoleezza Rice.

California GOP consultant Matt Rexroad, though, said he doesn't expect members of the GOP congressional delegation to run, saying that it's basically "pointless in terms of trying to win" and that members will prefer to keep their safe House seats rather than go up against the likes of Harris or Newsom.

Updated to reflect Garcetti saying he won't run. (h/t Reid Wilson)