LOS ANGELES — At 84, Sen. Dianne Feinstein is at an age when many Americans are deep into their retirement years. But the oldest member of the Senate announced Monday that she will seek another term with a ready retort for those who might suggest her best years are behind her.

“Experience counts,” the veteran California Democrat said.

The announcement that she would seek a fifth full term was expected and she has been stockpiling campaign dollars for a re-election bid. In recent months, the former San Francisco mayor has shown no signs that she was thinking about stepping aside for another generation.

With seats on several of the most powerful panels on Capitol Hill — the Judiciary, Appropriations and Intelligence committees — Feinstein has been a leading Democratic foil to President Donald Trump and his agenda.

As the top Democrat on the Judiciary Committee, Feinstein has focused, often in futility, on gun control issues and immigration. The Intelligence Committee has been conducting an inquiry into Russia’s election meddling and whether there was any collusion between Russia and Trump’s campaign.

In a statement, she said the Trump era has seen “democracy challenged in unacceptable ways,” from the attempts to roll back national health care for millions of people to attacks on reproductive rights and efforts to slow climate change.

“I cannot and will not step away from this fight,” she said. “There’s still so much work left to be done.”

Feinstein never directly referred to her age but argued her years in the Senate, where seniority carries clout, made her an effective voice for a state that is home to 1 in 8 Americans.

“The ability to get things done counts. And the compassion, vigor, and stamina to make a difference counts,” she added, seeming to anticipate questions about her age.

With another six-year term in Washington, Feinstein could be in the Senate into her 90s.

Questions about her advancing age circulated in 2012, when at 78 she was easily re-elected over token Republican opposition. She had a pacemaker installed in January.

Nonetheless, she enters the race the favorite to hold the seat she first won in 1992 in the strongly Democratic state. Democrats control every statewide office in California, dominate both chambers of the Legislature and hold a 3.7-million edge in registered voters.

In a state where Republican influence has withered, Feinstein’s biggest challenge next year could come from her own party. The break between the party’s establishment and liberal branches played out during last year’s Democratic presidential primary. And at a state Democratic convention this year, liberals inspired by Sen. Bernie Sanders, I-Vt., nearly captured the party’s top job.

Feinstein’s centrist credentials and lack of enthusiasm for universal health care have created friction with her party’s restless left wing. She’s had protesters outside her home, and has been criticized by some for appearing too temperate at times in remarks about Trump’s White House.

“I don’t think any progressive Democrat would be happy with the news” of her re-election bid, said Michael Thaller, a former chair of the California Democratic Party’s Progressive Caucus.

However, with no announced major candidates to challenge her, it’s not clear how big the threat might be. Recent polling suggests Feinstein is overwhelmingly popular with state Democrats. Although from Northern California, she has consistently carried crucial Los Angeles County, home to 10 million people. She has also displayed crossover appeal, pulling votes from a significant chunk of independents and even some Republicans.

Several big-name Democrats quickly lined up behind her.

California’s junior senator, Democrat Kamala Harris, wrote on her Facebook page that she strongly supported Feinstein’s bid for a fifth term. Los Angeles Mayor Eric Garcetti is hosting a fundraiser for her on Tuesday in Beverly Hills. Lt. Gov. Newsom used Twitter to announce his support, calling Feinstein “tough as nails.”