Race for U.S. Senate: Gavin Newsom out, Kamala Harris in

Montana Newsom, 5, second from left, looks up at her father, Lt. Gov. Gavin Newsom, as he takes the oath of office from Attorney General Kamala Harris, right, during an inauguration ceremony in Sacramento, Calif., Monday, Jan. 5, 2015. Along with Montana, Newsom was accompanied by his wife, Jennifer, and daughter Brooklynn,1. less Montana Newsom, 5, second from left, looks up at her father, Lt. Gov. Gavin Newsom, as he takes the oath of office from Attorney General Kamala Harris, right, during an inauguration ceremony in Sacramento, ... more Photo: Rich Pedroncelli / Rich Pedroncelli / Associated Press Photo: Rich Pedroncelli / Rich Pedroncelli / Associated Press Image 1 of / 7 Caption Close Race for U.S. Senate: Gavin Newsom out, Kamala Harris in 1 / 7 Back to Gallery

Lt. Gov. Gavin Newsom took himself out of the running for Barbara Boxer’s Senate seat on Monday, but state Attorney General Kamala Harris is all in.

Hours after Newsom said he has “unfinished work” in California and will not be a Senate candidate in 2016, a source close to Harris said she will announce Tuesday that she will run to replace Boxer.

“She is jumping in feet first — and she is in it to win it,” said the source, speaking on condition of anonymity.

The developments provide clarity not just to the 2016 Senate race, but possibly the 2018 governor’s campaign as well.

The two Democrats were widely seen as well-positioned to make a run for higher office in a state dominated by the Democratic Party. The question, once Boxer announced her impending retirement last week, was whether both would make a run for the Senate or whether at least one one would hold off until 2018, when Jerry Brown will be termed out as governor.

Newsom’s announcement “for all practical purposes represents the first step in his gubernatorial campaign,” said Dan Schnur, director of the Jesse Unruh Institute of Politics at the University of Southern California.

Bob Mulholland, a longtime California Democratic strategist, also said Newsom’s announcement suggests he has his sights set on replacing Brown. “I think you can read between the lines; he’s running for governor,” Mulholland said.

“It’s always better to be candid than coy,” Newsom said in a post on his Facebook page announcing his decision not to run for the seat held by fellow Democrat Boxer. “While I am humbled by the widespread encouragement of so many and hold in the highest esteem those who serve us in federal office, I know that my head and my heart, my young family’s future, and our unfinished work all remain firmly in the state of California — not Washington, D.C.”

Newsom told The Chronicle in a text message that the Senate “is not right for (my) family at this moment.”

No battle with Harris

Few political observers thought a Newsom-versus-Harris battle was in the cards. The two held office in San Francisco at the same time — Newsom as mayor, Harris as district attorney — and share many of the same political beliefs. They even have the same political consultant. Last week, Newsom chose Harris to swear him in for his new term in office.

“The conventional wisdom is that Newsom and Harris have decided that running against each other for either office (Senate or governor) doesn’t make much sense,” Schnur said.

The two did not cut a deal to divvy up campaigns, said the source close to the attorney general.

“She never talked to him,” the source said. “I think he read the tea leaves and made his decision.”

However, Newsom — who has long let it be known that he wants to be governor — said he talked to Harris on Sunday night to let her know that he wasn’t going to be running for Senate.

Harris is not expected to make a splashy appearance to announce her campaign.

“There will be a statement and some phone calls,” said the source close to Harris. “It’s a low-key launch and a long march to (the primary) next June.”

Although Harris is the biggest name so far to jump into the race, there’s no shortage of Democrats taking a look. Already, former Los Angeles Mayor Antonio Villaraigosa, Rep. John Garamendi of Walnut Grove (Sacramento County) and former Contra Costa County Rep. Ellen Tauscher have said they are considering a run.

Democrats in discussion

Other Democrats often mentioned as possible candidates include state Insurance Commissioner Dave Jones, state Treasurer John Chiang, former Southern California Rep. Jane Harman and environmental activist Tom Steyer, who spent $75 million last year in a largely unsuccessful effort to elect green-leaning Democrats nationwide.

“If Tom Steyer shows interest in this seat, his financial resources make him an immediate competitor,” Schnur said. “Tony Villaraigosa would bring a Southern California and Latino base into the race, which makes him very competitive. So Harris may start out from a position of strength, but there’s certainly no guarantee for her.”

East Bay Rep. Eric Swalwell is also considering a bid, said his campaign consultant Lisa Tucker. Swalwell would have to give up his House seat, but with Newsom out, his allies think the 34-year-old Democrat from Dublin would have a strong appeal to young voters.

Tucker said Swalwell is looking at the feasibility of raising the $20 million experts say would be needed for just the primary campaign.

Possible GOP contenders

On the Republican side, party activists see San Diego Mayor Kevin Faulconer, Fresno Mayor Ashley Swearengin, former state Insurance Commissioner Steve Poizner and former gubernatorial candidates Meg Whitman and Neel Kashkari as potential candidates.

Former California Republican Party Chairman Tom Del Beccaro, who is exploring a long-shot bid, said Boxer’s seat isn’t the only one potentially in play. He noted speculation that Sen. Dianne Feinstein will retire rather than run for re-election in 2018, when she will be 85.

Republicans need to “make an attempt at these seats regardless of the odds,” Del Beccaro said.

With Democrats enjoying a 43 to 28 percent advantage over Republicans in the state’s voter registration, the Senate seat should be theirs for the taking. But the party’s advantage is complicated by the state’s top-two primary system, which pits the No. 1 and No. 2 vote-getters against each other in the general election, regardless of party affiliation.

Schnur counted two dozen instances since the new system was adopted for the 2012 elections in which two members of the same party ran against each other for seats in Congress or the Legislature. The biggest challenge for Democrats, Schnur said, “is not overcrowding the field with so many candidates that it creates an opportunity for a Republican” to win a spot in the general election.

Phillip Matier, Andrew Ross and Carolyn Lochhead are San Francisco Chronicle staff writers. E-mail: matierandross@sfchronicle.com, clochhead@sfchronicle.com Twitter: @matierandross, @carolynlochhead