Maryland Sen. Barbara Mikulski, the longest serving woman in congressional history, will not run for a sixth term in the U.S. Senate in 2016, her office announced on Monday. The 78-year-old Democrat said her decision came down to how the demands of another campaign would have affected her ability to work on behalf of her constituents.

"I had to decide whether to spend my time fighting to keep my job or fighting for your job," Mikulski said. "Do I spend my time raising money or raising hell to meet your day-to-day needs? Do I spend my time focusing on my election or the next generation?"

Mikulski, who had led the powerful Appropriations Committee before the Democrats lost control of the Senate in the 2014 midterm elections, "for years been a force to be reckoned with on Capitol Hill," The Associated Press reported. The senator was not known for "social niceties" but won over Maryland voters by forcefully defending her constituents' interests.

The Democrat was first elected to the House of Representatives in 1976 and ascended to the Senate in 1987. She was the her party's first woman senator elected in her own right and the longest-serving woman in the body's history, her office detailed.

The AP called her a "role model to other female senators, Republican and Democrat" and said she was credited with getting rid of the chamber's requirement that women wear skirts on the floor.

"When Senator Mikulski decided that she was going to wear pants while casting her votes, it was the rule that had to change, not her," said Washington Sen. Patty Murray, a fellow Democrat. "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."

Mikulski's retirement might lead to a "raucous fight" to fill Maryland's first open Senate seat in a decade, the AP speculated. A prime contender for the spot, former Gov. Martin O'Malley, meanwhile, said he would not run for the Senate in 2016, the Baltimore Sun reported. O'Malley is said to have presidential ambitions.

Other likely candidates to succeed Mikulski include another former governor, Republican Bob Ehrlich, as well as current Reps. Elijah Cummings, Donna Edwards, Andy Harris, Dutch Ruppersberger and Chris Van Hollen.