Merely a week into 2015, California’s 2016 election season began with a bang Thursday as U.S. Sen. Barbara Boxer — one of the Senate’s most steadfast liberals — announced she won’t seek re-election next year.

Boxer’s decision not to seek a fifth six-year term sets off what is likely to be a wild succession struggle. Names being bandied about as potential candidates include politicos like Lt. Gov. Gavin Newsom, Attorney General Kamala Harris and former Los Angeles Mayor Antonio Villaraigosa; business figures such as hedge fund billionaire and environmental activist Tom Steyer; and Republicans such as former HP CEO and 2010 challenger Carly Fiorina.

The big question is which of these potential candidates will opt to seek Boxer’s seat in 2016, and which will wait for 2018 to run to succeed Gov. Jerry Brown or perhaps U.S. Sen. Dianne Feinstein, 81, should she also choose to retire.

Boxer, 74, announced her decision with a video in which she’s “interviewed” by her grandson, Zach Rodham, 19. She said neither the constant fighting in the Senate — which she relishes — nor her age was a factor in her decision.

“Some people are old at 40 and some people are young at 80 — it depends on the person. As for me, I feel as young as I did when I got elected,” she said in the video. “I am never going to retire, the work is too important. But I will not be running for the Senate in 2016.”

Instead, she’ll “continue working on the issues that I love; I’ll have more time to help other people through my PAC for Change community, I have to make sure this Senate seat stays progressive — that is so critical — and I want to help our Democratic candidate for president make history,” she said. “But you know what? I want to come home. I want to come home to the state that I love so much, California.”

She also offered her decision in rhyme.

“The Senate is the place where I’ve always made my case — for families, for the planet and the human race,” she said. “More than 20 years in a job I love, thanks to California and the Lord above. So although I won’t be working from my Senate space and I won’t be running in that next tough race, as long as there are issues and challenges and strife I will never retire because that’s the meaning of my life.”

A Brooklyn, N.Y., native, Boxer worked as a stockbroker before moving to California, where she worked in the 1970s as a journalist for the alternative weekly Pacific Sun newspaper and as an aide to U.S. Rep. John Burton. She served on the Marin County Board of Supervisors from 1976 to 1982, when she was elected to succeed Burton in Congress.

 Barbara Boxer, 2013.

She won her Senate seat in 1992, deemed “The Year of the Woman,” when she was one of four female candidates to win Senate seats that year. Long considered one of the Senate’s most liberal members, she might have made her mark most deeply as chairwoman of the Environment and Public Works Committee, which has dominion over everything from highway bills to nuclear safety to climate-change issues.

Her statement notwithstanding, Boxer’s “sheer frustration” over Democrats’ loss of the Senate majority might have played a big role in Boxer’s decision, said Sherry Bebitch Jeffe, a University of Southern California senior scholar and political expert.

“It’s not only that Democrats are in the minority, but that she has lost her committee chair to (Oklahoma Sen.) James Inhofe, who denies global warming,” Jeffe said. “She has lost whatever power she has wielded on that issue, and that’s tough.”

Jeffe also noted that Boxer might not fare as well in the minority as Feinstein, the latter of whom has developed ties and trust across the aisle and developed a reputation as a moderate. “At least the public perception, and perhaps the reality, is that Barbara Boxer hasn’t done that.”

Feinstein said she and Boxer “worked particularly hard on the fight against global warming — I think no one is more engaged on this issue than she. It’s been an uphill battle, but today we’re seeing the success Barbara has had on making climate change a real priority for Americans.”

“She is never one to shy away from any challenge, and I can’t thank her enough for being such a resilient collaborator,” Feinstein added. “We blazed many trails together, and now I’m eager to see where her next steps take her.”

At her weekly news conference, House Minority Leader Nancy Pelosi described Boxer as “one of the most unselfish politicians I have ever known of,” sharing ideas and cooperating across the aisle. “Her leaving will be a great loss to the Congress of the United States, the people of California, and to our country.”

Jeffe said there’s buzz “that Gavin Newsom and Kamala Harris will sit down and decide who gets to go for Senate and who gets to go for governor. But I do recall that this has happened before — and whenever that kind of a deal is made, it ends up badly for both actors in the deal.”

Most notably, the “big switch of 1958” in which U.S. Sen. William Knowland and Gov. Goodwin Knight struck a deal to run for each other’s seats led to both Republicans being defeated. “When something visibly manipulative happens, voters don’t like it,” Jeffe said.

Newsom issued a statement Thursday saying Boxer “with the courage of her convictions and the unflinching spirit of a true warrior-advocate … has earned a seat in the pantheon of great California leaders.” She was his county supervisor when he was growing up in Marin County, he noted, so he learned early on that “there’s no ‘quit’ in Barbara Boxer. While she might leave the Senate, she will never retire from her leadership on the progressive causes she’s led for decades.”

Steyer lauded Boxer as “a warrior for progressive causes — protecting our climate, championing the rights of women and children, and defending the core Democratic values of civil rights and economic equality for millions of Americans. We Californians have been very, very fortunate to have her representing us.”

Staff writer Jessica Calefati contributed to this report. Josh Richman covers politics. Follow him at Twitter.com/Josh_Richman. Read the Political Blotter at IBAbuzz.com/politics.