A proposal to expand equal-pay protections for female workers sponsored by Sen. Barbara A. Mikulskifailed to clear a procedural vote in the Senate on Tuesday, ending the measure's chances but opening a potential Democratic attack line in this year's election.

The Paycheck Fairness Act, which would have expanded the 1963 Equal Pay Act by prohibiting employers from firing workers who discuss their salary with their colleagues, fell seven votes shy of the 60 needed to bring the legislation to the Senate floor.

Democrats knew the legislation faced long odds when they reintroduced it this year. But they hope to use the bill's failure as a political weapon against Republicans -- who opposed it unanimously -- as candidates in both parties court female voters ahead of the November election.

"We want to go out and tell the story of what happened here today," Mikulski, a Maryland Democrat and the longest-serving woman in Congress, said minutes after the vote. "We want to tell not only the women of America about this but also the good men who support them."

In addition to prohibiting companies from banning colleagues from talking about salary, the legislation limits circumstances under which an employer can pay men and women differently. And it lets women sue employers for punitive damages if they can demonstrate they were paid unfairly.

Republicans zeroed in on that last provision, arguing that the proposal would be a boon for trial lawyers. "We don't think America suffers for a lack of litigation," Senate Leader Mitch McConnell of Kentucky said in explaining his opposition.

The measure had also been opposed by business groups, including theU.S. Chamber of Commerce.

Fifty Democrats and two independents voted in favor of the bill. Senate Democratic Leader Harry Reid of Nevada supports the bill but voted against it -- a procedural move that allows him to bring it up again.

Supporters, including President Barack Obama, launched an aggressive public relations blitz in the days leading up to Tuesday's vote.

"It is incredibly disappointing that in this make-or-break moment for the middle class, Senate Republicans put partisan politics ahead of American women and their families," Obama said in a statement following the vote.

Democrats renewed calls for Republican presidential candidate Mitt Romney to articulate a position on the bill. The Romney campaign has responded to questions about the Mikulski proposal by pivoting to the economy at large.

"Of course Governor Romney supports pay equity for women," Romney campaign spokeswoman Amanda Henneberg said in a statement. "In order to have pay equity, women need to have jobs, and they have been getting crushed in this anemic Obama economy, losing far more jobs than men."

Women take home about 77 cents for each dollar earned annually by men, according to the Institute for Women's Policy Research, a Washington think tank. Some of that difference is based on the types of occupations men and women often choose. But 25 percent to 40 percent of it is caused by discrimination, the institute estimates.

The Paycheck Fairness Act is the latest measure moving through Congress this election year. The House and Senate are also battling over different versions of a bill that that would spend $660 million a year over five years on combating domestic violence.

Lilly Ledbetter, who sued Goodyear in 1998 for pay discrimination in a case that went to the Supreme Court, was among the advocates on Capitol Hill Tuesday to lobby for the bill.

Ledbetter was the inspiration for another measure sponsored by Mikulski that extends the statute of limitations for suing an employer for wage discrimination. Congress approved that bill in 2009 and it became the first major piece of legislation Obama signed into law.