WASHINGTON — Sen. Barbara Boxer, a California Democrat who is one of the most forceful liberal voices in the Senate, said Thursday she will not seek re-election in 2016.

Boxer, 74, said in a video posted on her website that she will focus on making sure a Democrat is elected to replace her and on the 2016 presidential race. She is the first senator to announce retirement ahead of the 2016 elections.

“The Senate is the place where I’ve always made my case, for families, for the planet and the human race,” Boxer said in a rhyme written for the announcement. “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, challenges and strife, I will never retire, because that’s the meaning of my life.”

Boxer will use PAC For a Change, her fundraising committee, to continue to support Democratic candidates, according to the video.

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 during 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.”

“There will be real recognition of the difference she has made for fairness in our economy, protection of our environment, respect for our men and women in uniform,” Pelosi told a weekly news conference.

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.

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.

Democrats who could seek to replace her include California Attorney General Kamala Harris and state Lt. Gov. Gavin Newsom, Jeffe said. A Republican candidate would be unlikely to win the seat in heavily Democratic California, she added.