From Sen. Barbara Boxer's Instagram account

When Sen. Barbara Mikulski (D-Md.) took the Senate oath in 1987 she was immediately all by herself in the history books, becoming the first Democratic woman elected to a Senate seat in her own right. (Typically, women had come to hold elected office when their husbands died.  Not so for Mikulski, who announced her retirement Monday morning.)

In 2012, when she became the longest serving woman in the Senate, she had much more company--19 other women were in the Senate too. Call it the Mikulski effect.

"The Senate used to be a lonely place for women, but Senator Mikulski changed that," wrote Sen. Barbara Boxer, who is also retiring in 2016. "For the women of the Senate, she will always be our Dean -- the woman who opened up the doors of the Senate wide enough to let the women of America walk in. The people of Maryland have been lucky to have such a champion -- a 4' 11'' fighter for the middle class, for women, for families, for seniors, for veterans"

Much has changed in America -- and the Senate --  since Mikulski first ran. Her stature and her gender made some people comment that she didn't "look Senatorial," a framing she recognized for exactly what it was. "A lot of Americans, black or white or female, are always told that they don’t look the part. It’s one of the oldest code words," she told Time Magazine.

Once she won, Mikulski, with her very presence, changed long-held ideas about what Senators should look like. And what women should wear.  Mikulski and her fellow Senator Nancy Kassebaum (R-Kan.) mounted a protest one weekend that amounted to something rather simple: they wore pants and told female staffers to do the same. From then on, the rule changed and the pantsuit became routine, but it didn't go over so easy. "The Senate parliamentarian had looked at the rules to see if it was okay," she said in a CNN interview. "So, I walk on that day and you would have thought I was walking on the moon. It caused a big stir."

It was 1993.

"She fought for us before we even got here, walked into rooms women had not been welcome in before, and made sure to keep her foot stuck in the door so other women could walk in behind her," Sen. Patty Murray (D-Wash.) said in a statement. "Every women in the Senate today, and women in other positions across the country, owes her a tremendous debt that can never be fully repaid."

The Mikulski effect can be seen locally as well as nationally in the names floated to replace her: Baltimore Mayor Stephanie Rawlins Blake, Rep. Donna Edwards, and Heather Mizeur, the first openly gay woman to run for Maryland governor.

Sen. Barbara Mikulski (D-Md.), the "dean" of Senate women, announced she won't seek reelection in 2016. Here are some of her most memorable moments from her time on Capitol Hill. (Pamela Kirkland/The Washington Post)