Barbara Mikulski chats with a group of women senators and senators-elect in November 2014.Jonathan Ernst/Reuters

When Maryland's Barbara Mikulski steps down in January 2017, she'll close a chapter in Senate history. The diminutive Democrat, who will announce her decision not to run for reelection Monday, holds a passel of congressional records, including as the longest-serving woman in Congress and the longest-serving female senator.

At 78, Mikulski's retirement isn't a total surprise. But the change between the Senate she joined in 1987 and the Senate today is a little jarring. "Women were so rare even holding statewide political office," she told The Washington Post in 2011. "I was greeted with a lot of skepticism from my male colleagues. Was I going to go the celebrity route or the Senate route? I had to work very hard."

There was only one other woman in the chamber; Mikulski was only the 16th in history, and many of her predecessors had entered office through circumstances like filling their deceased husbands' seats. Senator Nancy Kassebaum noted to Mikulski that women weren't allowed in the Senate gym.

The Baltimore native has since become the dean and convener of women in the Senate. As the number of women in the body has grown, she's worked to gather senators from both parties—a rare point of bipartisan contact, and one that the women says makes them better dealmakers.

The progress on women's representation since Mikulski—already a five-term U.S. representative—won election to the Senate in 1986 is both striking and underwhelming, depending on your perspective. There are 20 women in the Senate, the most ever, but that still lags far behind the demographics of the general population. And as late as 2008, the Senate pool was male only—because, Liza Mundy reported, some male members liked to swim nude.

"I would have a more 'glass is half empty' outlook on this one," Jennifer Lawless, director of the Women & Politics Institute at American University, told the Post in 2011. "When we are 52 percent of the population and only 17 percent of the Senate, it's difficult to say we've made so much progress, especially when that number has plateaued."

With California Senator Barbara Boxer's announcement earlier this year that she, too, will not run for reelection, the old guard among women is fading. But there is a large crop of younger female senators, ranging the political spectrum from New York Democrat Kirsten Gillibrand to Iowa Republican Joni Ernst.

In the Senate, Mikulski fought for equality issues in the Senate and has been a reliable liberal. She was perhaps the original Senate Democratic community organizer, rising to prominence as a champion of ethnic communities in the U.S. Coming from a strong Polish background, she argued that white working-class ethnics were pushed around by the political system, accused of racism, and discriminated against by white liberals.

Mikulski's retirement should offer an opportunity for Maryland Democrats to move up. (The Bay State's other senator, Democrat Ben Cardin, was elected in 2006 after a long stay in the House, where he replaced Mikulski.) Since losing her first bid for Senate, in 1974, Mikulski has never won less than 60 percent of the vote. But Democrats were stunned in November when their candidate for governor, Anthony Brown, lost to Republican Larry Hogan.

There should be no shortage of candidates. Former Governor Martin O'Malley continues to flirt with what almost every analyst thinks is a quixotic bid for the 2016 Democratic presidential nomination against Hillary Clinton. Opting for the Senate instead might offer him a graceful alternative. His 87-year-old mother Barbara O'Malley has served in Mikulski's office since she joined the Senate. Chris Van Hollen, a former member of the House Democratic leadership and rising star, is also being mentioned a natural candidate. The New York Times' Jonathan Martin notes that the state could be demographically primed to elect a black senator.

At 4-foot-11, it would perhaps be misleading to say that she'll leave large shoes to fill. But replacing Barbara Mikulski will be a tall order.

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