The Most Exciting Senate Race of 2016 Starts Now

A handful of Democrats are already vying for Barbara Boxer's not-yet-empty seat.

The most expensive, hard-fought, and interesting Senate race in the country next year might happen in California—and it could be confined entirely to one party.

Sen. Barbara Boxer's retirement announcement Thursday didn't come as a surprise to most people, least of all to a handful of Democrats who had already begun publicly and privately angling for her job. Even before her retirement, party officials had cobbled together an unofficial short list of possible successors, including former Los Angeles Mayor Antonio Villaraigosa, billionaire environmental activist Tom Steyer, California Attorney General Kamala Harris, and Lt. Gov. Gavin Newsom.

But for all the intrigue that Boxer's impending departure had already stirred among Democrats, it's unlikely to change the calculus behind the party's larger strategic goal of retaking control of the Senate in 2017. Not only is a Democrat likely to win the 2016 presidential race in California, it's possible there won't even be a Republican on the general-election ballot in the Senate race.

There's been considerably less buzz about potential Republican candidates, and with good reason. California is famously liberal, overwhelmingly Democratic at the federal level, and entering a presidential year where the last Republican presidential nominee lost by 23 points.

But perhaps most important, the state's top-two primary system means that a Republican candidate isn't even guaranteed to make it to the general election. (Instead of traditional primaries, the top pair of vote-getters next June will make the fall ballot, even if they share a party.)

Republicans could muster a wealthy, top-flight recruit, and in a field with a half dozen or more serious Democratic candidates, the GOP hopeful could reach the fall election with a relatively small share of the vote. But even Republican officials in Washington downplayed their chances. The National Republican Senatorial Committee chose to focus on how Boxer's departure signals broader problems for Democrats—not the party's opportunity to win in California.

"This could be the first of many Senate retirements thanks to their new Democrat minority status," said NRSC spokeswoman Andrea Bozek in a statement. "Today's news raises the question whether there will be additional Democrat Senate retirements on the horizon. Senate Democrats are already $20 million in debt, and Senator Boxer's retirement can't be welcome news for the DSCC who has to defend a costly and hotly contested open seat."

The absence of a serious Republican candidate would raise the possibility of an intra-Democratic fight that lasts not just until next June's primary, but all the way through the fall. And for a party that could miss an opportunity to fight over its future in next year's presidential primary, a Senate primary race in California could give its centrist and populist wings a chance to do battle in what would be a J.V. version of a national contest.

Just don't expect the race to include both Newsom and Harris, two young, telegenic politicians both thought to be rising stars within the party. Although they are considered possible candidates, few expect the pair, who are friends, to run against each other.

"Gavin's perspective is let her run in 2016, and he'll be biggest cheerleader," said a California Democratic strategist, who emphasized that the lieutenant governor is likely more interested in running for governor in 2018. "If she wins, that part of the equation is done. There are other players out there, but that removes the biggest question."

Scott Bland contributed to this article