Boxer said Thursday she won't seek re-election after 4th term. (AP Photo/Manuel Balce Ceneta, File)

Sen. Barbara Boxer (D-Calif.) announced on Thursday that she will not seek reelection in 2016, the first retirement announcement from a Democratic senator ahead of the 2016 political cycle that will spark a major political contest in California.

Boxer made the announcement in a video co-starring her grandson, who played the role of reporter.

In an interview with her grandson, Sen. Barbara Boxer (D-Calif.) announced that she will not seek reelection after her fourth term. (Barbara Boxer via YouTube)

Boxer was elected to the House in 1983 and won her Senate seat in 1992 — the so-called "Year of the Woman." She has served alongside the Golden State's senior senator, Dianne Feinstein (D-Calif.), making up one of the first all-woman Senate teams from any of the 50 states.

“I always knew I had a partner in Barbara. She is never one to shy away from any challenge, and I can’t thank her enough for being such a resilient collaborator,” Feinstein said in a statement Thursday. “We blazed many trails together, and now I’m eager to see where her next steps take her. Barbara is so passionate about so many things, I know her work has really just started. I’m sure she’ll continue to be a role model and inspiration to us all.”

Boxer, who in recent years served as head of the Senate Environment and Public Works Committee, has been a reliable liberal voice in the often-fractured chamber.

Rumors of her potential retirement had been swirling since late last year, and recent polling suggested her popularity was in decline, despite being from a deep-blue state. A recent statewide poll gave her a 41 percent approval rating. A third-quarter Federal Election Commission filing showed that Boxer had just under $149,000 cash on hand, indicating that she was not aggressively fundraising for reelection.

Boxer's retirement sets up what could be an epic, protracted and incredibly expensive Democratic primary to replace her — in a state that hasn't had an open Senate race since George H.W. Bush was president. Rumored candidates include Lt. Gov. Gavin Newsom, Attorney General Kamala Harris, Rep. Jackie Speier (D-Calif.), billionaire climate activist Tom Steyer, current Los Angeles Mayor Eric Garcetti and former Los Angeles mayor Antonio Villaraigosa.

Under questioning by her grandson, Boxer, 74, said in the video that the protracted nature of the Senate "is not a factor in my decision."

Nor is her age.

"No, definitely not. ... Some people are old at 40, and some people are young at 80. ... As for me, I feel as young as I did when I was elected. I was in my 50s when I was elected to the Senate," she said.

"I want to come home," she added later, before concluding the video with a rhyme. (She's frequently used limericks as a rhetorical tool.)

"The Senate is the place where I’ve always made my case," she said. "For families, for the planet and the human race. More than 20 years in a job I love, thanks to California and the Lord above. So although I won’t be working for 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."

News of Boxer's retirement appeared to catch her close friend, House Minority Leader Nancy Pelosi, a fellow California Democrat, by surprise.

"What?" she responded when asked by reporters about it Thursday morning during her weekly news conference.

House Minority Leader Nancy Pelosi (D-Calif.) told members of the press on Thursday that the decision by Sen. Barbara Boxer (D-Calif.) to not seek reelection would be "a great loss" to the American people. She also spoke of the two women's close personal relationship. (AP)

Pelosi said she had just received a call from Boxer indicating she wanted to speak with her personally.

"I thought she wanted to have dinner tonight," Pelosi quipped.

Pelosi said Boxer's decision would mean "a real loss" but added, "God bless her."

"Senator Boxer has been such a champion for the people for California and indeed for our entire country," she added.

Jose A. DelReal and Sean Sullivan contributed to this report.