Sen. Barbara Boxer (D-Calif.), shown here in March 2014, announced she will not run for re-election in 2016. (Chip Somodevilla/Getty Images)

Nearly 24 years ago then-Rep. Barbara Boxer (D-Calif.) helped lead a group of seven female Democrats across the Capitol hoping to crash one of the most exclusive clubs in America, a closed-door luncheon of Senate Democrats.

The country was in the midst of a great political skirmish over sexual harassment, set off by the accusations of misconduct against Supreme Court nominee Clarence Thomas.

She and the other women were denied entry into that lunch in October 1991, and were unable to deliver in person their objections to how the matter was being handled. But they helped compel the Senate into a heated set of confirmation hearings that gave the issue an international profile.

A year later, in what became know as the Year of the Woman, Boxer won a Senate seat as part of a sweeping new group of female lawmakers that went to the Capitol. Together, they put Congress on a slow but steady course of change in how it considered laws that had a disproportionate impact on women.

Boxer, now one of 20 female senators, announced Thursday that she would not seek reelection in 2016. The news was expected, given that she had barely raised any campaign money in a state that requires tens of millions of dollars to run a Senate campaign.

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)

But the announcement by Boxer, 74, signaled that members of this historically important generation of female lawmakers is approaching careers’ end, and a new generation must step forward on women’s issues.

“It’s hard to think of another member of the Senate who is as strong an advocate for women’s rights, at so many different levels. On the issue of choice, she is the first up and the leader of the battle,” said Sen. Richard J. Durbin (D-Ill.), one of Boxer’s closest friends who entered the House with her in 1983.

Her departure will also end nearly a quarter-century of stability in the California’s delegation to the Senate, where Boxer and fellow member of the class of 1992, Sen. Dianne Feinstein (D), have racked up nearly 50 years of combined experience, chairing several critical committees.

In line behind them are nearly a dozen Democrats who have been waiting for a chance at a Senate seat. Boxer’s departure has set off a wild race to see who will succeed her. The seat is most likely to stay in Democratic hands — in her video announcement Boxer said one of her last goals was to assure a “progressive” replaces her — but the state’s odd, relatively new “jungle primary” system could allow a wealthy independent from Silicon Valley or from Southern California to make it into the November general election.

The “jungle primary” dispenses with party affiliation; primary voters can cast their ballot for any candidate of any party in the summer primary, sending the two top vote-getters into the November general election.

So in a state with a lot of pent-up Democratic ambition, Democratic voters could split their votes, sending a non-Democratic into the general.

After more than two decades without an open seat, the race could attract a lot of candidates and a lot of money, likely costing more than $100 million among all the candidates and outside groups.

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)

The two most watched figures at the moment are Lt. Gov. Gavin Newsom and state Attorney General Kamala Harris, two Democrats who have already won statewide office and might have the connections to raise enough money to compete in a state where grassroots campaigning doesn’t reach enough voters.

On Thursday, one senior Democratic operative suggested that both Harris and Newsom would not run because of their close political ties and that, at the moment, “Kamala will have a head start.” Feinstein, 81, may not run for reelection in 2018, when Gov. Jerry Brown’s term will be up, setting up the possibility of three blockbuster statewide races within two years.

Other potential Democratic candidates include former Los Angeles mayor Antonio Villaraigosa, San Jose Mayor Sam Liccardo, Rep. Loretta Sanchez, state Insurance Commissioner Dave Jones, state Treasurer John Chiang, former congresswoman Jane Harman and Rep. Karen Bass.

Living up to Boxer’s liberal legacy will be a challenge for the Democratic aspirants, although recent polls have suggested that her staunch ideology had left her unpopular in the vast rural regions of the state. She has previously been the chairman of the Senate Environment and Public Works Committee, overseeing climate policy and also highway funding.

For the last eight years she has also chaired the Ethics Committee, a post that at times has brought her face-to-face with the sort of sexual misconduct allegations that propelled her into the Senate after a decade in the House.

Back in 1991, Boxer’s campaign seemed like a longshot. There were just two female senators. Boxer went to Sen. Barbara Mikulski (D-Md.) for advice about the race, and Mikulski encouraged her to jump in.

On the night in June 1992 when she won the primary, over another congressman and the sitting lieutenant governor, she declared what kind of senator she would be: “They want a fighter in the Senate, don’t they? They want someone who will shake up the Senate, don’t they? I will be a fighter in the Senate.”

Boxer narrowly won the general election, part of a group of four women — Boxer and Feinstein, along with Patty Murray of Washington and Carol Moseley-Braun of Illinois — to join the Senate that year. In each case it was the first time their state had elected a woman to the Senate. That same election increased the number of women in the House from 28 to 47.

Most of those new female lawmakers were Democrats, and the Thomas hearings for the Supreme Court played a galvanizing role in inspiring women’s votes and inspiring female candidates. Thomas had been accused by University of Oklahoma law professor Anita Hill of inappropriate sexual conduct dating back to when they worked together at the Equal Employment Opportunity Commission.

Boxer and other House Democrats, including Reps. Rosa DeLauro (D-Conn.) and Nita Lowey (D-N.Y.), helped push Hill’s allegations into a higher profile after a private interview of Hill by the FBI had not received attention from the all-male Judiciary Committee.

In the early years in office Boxer, Feinstein and Sen. Patty Murray (D-Wash.) set a tone that their colleagues would have to behave differently, and soon after the chamber’s ethics committee was investigating sexual misconduct by then-Sen. Bob Packwood (R-Ore.). He eventually resigned.

Boxer’s style, often seemed inspired by her last name, could grate on Republican colleagues who sometimes felt as if she wanted a fight more than a legislative result. “I’ve been ringside with the Boxer battles for many years. I just have so much affection and admiration for her,” Mikulski recalled Thursday.

“She certainly has been an effective voice for liberal causes in this country, and believes passionately in her views, and she will leave a void on that side of the aisle,” said Sen. Susan Collins (R-Maine), who won election in 1996 and is now the most senior female Republican senator.

Throughout the 1990s and into the early 2000s, the number of female senators inched upward but the Senate could still be a lonely place. Collins became chairman of the Homeland Security and Government Affairs Committee in the early days of George W. Bush’s administration.

“I remember looking out one day, because Donald Rumsfeld was testifying before us on some personnel issues, and seeing this mass of men, and then looking to my left and to my right on the panel, all men. And how it has changed,” said Collins, who noted that last year every Senate committee had at least one female member.

Boxer is ready for one last major political fight: She declared Thursday that she would be a champion for Hillary Rodham Clinton’s likely presidential campaign. She broke her retirement news in a well-produced video in which her grandson — Zach Rodham, who is also the nephew of Clinton — interviewed Boxer and asked what her remaining goals were.

“I want to help our Democratic candidate for president make history,” Boxer responded.

Aaron Blake and Ed O’Keefe contributed to this report.