U.S. Senator Barbara Boxer will not seek re-election

WASHINGTON (Reuters) - U.S. Senator Barbara Boxer of California, one of the Senate's most forceful liberal voices, will not seek re-election in 2016, making her the first of three top Democratic state officials over age 70 to announce plans to leave office.

It was not yet clear which of several ambitious younger Democratic politicians would make a bid for the seat Boxer will leave after more than three decades in the U.S. Congress. Other top California offices should open up in a few years. Senator Dianne Feinstein, 81, and newly re-elected Governor Jerry Brown, 76, are serving terms that end in 2018.

"The Senate is the place where I've always made my case, for families, for the planet and the human race," Boxer, 74, said in a rhymed announcement in a video posted on her campaign website Thursday.

Boxer is the first senator to announce plans to step down ahead of the 2016 elections. She said she plans to return to California and work to make sure a Democrat is elected to replace her and on the 2016 presidential race.

She stressed that she is not retiring from public life.

"I am never going to retire," she said. "The work is too important."

Two popular Democrats, Attorney General Kamala Harris, 50, and Lt. Governor Gavin Newsom, 47, are widely believed to be considering runs for one of the three top offices in this lopsidedly blue state where their party holds all statewide elective offices.

A spokesman for Newsom said that Thursday "was Boxer's day," and the Lt. Governor would not be discussing his future plans. Harris' political consultants did not immediately respond to requests for comment.

Los Angeles Mayor Eric Garcetti, 43, also eyed as a potential candidate to replace Boxer, said on Thursday he would not run.

Political analyst Sherry Bebitch Jeffe, a professor at the University of Southern California, said there had been speculation Boxer might not run because she had not engaged in serious fundraising.

Jeffe said a Republican candidate would be unlikely to win the seat in heavily Democratic California.

Boxer said in the video that she would use PAC For a Change, her fundraising committee, to support Democratic candidates.

Boxer has served in the Senate since 1993 and was in the U.S. House of Representatives for a decade before that. She has been a leading voice for environmental protection and chaired the panel that handles such issues, but lost that post after Republicans took control of the Senate in November's mid-term election.

House Democratic leader Nancy Pelosi, also of California and a close friend, described Boxer as "small in size but a giant in terms of her contributions to our country."

The senator stands about 5 feet tall and at public speeches has been known to stand on a small box nicknamed the "Boxer box."

Pelosi appeared to be surprised by the timing of Boxer's announcement. She said Boxer had put in a call to her earlier on Thursday to discuss something.

“I thought she wanted to have dinner tonight,” Pelosi said.

(Reporting By Amanda Becker and Richard Cowan in Washington and Sharon Bernstein in Sacramento; Additional reporting by Alex Dobuzinskis in Los Angeles; Editing by Doina Chiacu, Bill Trott, Andre Grenon and David Gregorio)