WASHINGTON — Sen. Barbara A. Mikulski is calling on the Obama administration to prohibit federal contractors from retaliating against employees who discuss salaries — a new twist on a key provision of the pay equality legislation the Maryland Democrat has sought for years.

In a letter to the White House on Tuesday, Mikulski argued that President Obama should sign an executive order baring contractors from firing employees who disclose their own pay or inquire about another employee’s salary.

Mikulski has sponsored legislation, the Paycheck Fairness Act, that would put that prohibition in place for all employers. But the measure fell seven votes shy of the 60 needed to bring it to the Senate floor last year amid a charged presidential election in which women voters were courted by both Obama and Republican nominee Mitt Romney.

She recently reintroduced the bill.

“Right now in the marketplace it is legal to fire a woman if she asks about her pay, whether she goes to the personnel director or if she asks the person next to her at the water cooler,” Mikulski said in a statement. “While we work to pass the Paycheck Fairness Act in Congress, we have to make sure that contractors doing business with the United States government are not retaliating against hardworking Americans for discussing salary information.”

A White House spokesman was not immediately available to respond to the letter.

Mikulski’s legislation would expand the landmark 1963 Equal Pay Act, which prohibits wage discrimination based on gender. The proposal would limit the circumstances under which an employer can legally pay men and women differently. It also would let women sue employers for punitive damages if they can demonstrate they were treated unfairly — a sticking point for business groups.

Mikulski is one of her party’s most ardent advocates on women’s issues. Four year ago, she successfully pushed through a law that extended the statute of limitations for suing an employer over wage discrimination — the Lilly Ledbetter Fair Pay Act. The bill was the first major piece of legislation Obama signed into law when he began his first term.

But she has had less success with the pay equality bill, which fell two votes shy of the 60 needed to advance when it last came up for a vote in 2010. In the past, the bill has failed to win any Republican co-sponsors.

Women take home 77 cents for each dollar earned annually by men, according to the Institute for Women’s Policy Research, a Washington think tank. Advocates believe part of that gap is based on differences in the types of occupations men and women choose and part is caused by discrimination.

An executive order could have a particularly profound impact in Maryland, home to many companies that do business with the federal government. Private government contractors in the state were awarded about $27 billion in work last year — among the highest per capita in the nation.

Obama raised pay equality as an issue in his inaugural address last week, but offered no specifics.

“Our journey is not complete until our wives, our mothers and daughters can earn a living equal to their efforts,” he said.