Republicans concentrated on the economic damage that they said the gender equity bill would inflict. They were backed by the U.S. Chamber of Commerce and other business groups.
"At a time when the Obama economy is already hurting women so much, this legislation would double down on job loss all while lining the pockets of trial lawyers," said Senate Minority Leader Mitch McConnell, R-Ky. "In other words, it's just another Democrat idea that threatens to hurt the very people it claims to help."
Democrats pushed the same legislation the last two election years, 2012 and 2010, only to see Senate Republicans scuttle the measures.
Mikulski's bill is aimed at tightening the 1963 law that made it illegal to pay women less than men for comparable jobs because of their gender.
It would shrink the loopholes employers can cite to justify such discrepancies and prevent them from punishing workers who share salary information. It would also make class-action lawsuits about paycheck unfairness easier and allow workers to seek punitive and compensatory damages.
Wednesday's vote was 53-44 for debating the legislation seven fewer than Democrats needed to keep the bill moving forward. Every voting Republican was against continuing work on the measure.
Sen. Angus King, I-Maine, who usually aligns with Democrats, voted with the GOP. He said later the bill ignored the real reasons for the pay gap between genders, such as companies that make it hard for women with children to continue working.
Senate Majority Leader Harry Reid, D-Nev., switched to vote against the legislation a maneuver that makes it easier for him to demand a future roll call on the bill. Top Democrats have promised to force Republicans to vote again on the issue before November.
"This won't be the last time they have to go home to their constituents and explain that they don't think this is a worthy issue," said Sen. Patty Murray of Washington, a member of the Senate Democratic leadership.
Sen. Lisa Murkowski, R-Alaska, said she opposed the Democratic bill because "perhaps this is more an exercise in political messaging than an effort to try to resolve what I believe is an issue."
Women consistently vote more often for Democrats than men do. They tilted Democratic in every election since 1976 but two: 2002 and 2010. In those two elections women divided about evenly, even as Republicans picked up congressional seats.
Women averaged 77 percent of men's earnings in 2012, according to Census Bureau figures.