By a vote of 56 to 42, the Senate fell four votes short of the 60 required to begin consideration of the Lilly Ledbetter Fair Pay Act, named for an Alabama woman who lost a case against the Goodyear Tire and Rubber Company when the court found she not did file her complaint in time. Ms. Ledbetter had been paid as much as 40 percent less than her male counterparts doing the same job, according to her allies.

The political elements of the debate were unmistakable. Unable to reach an agreement with Republicans, Democrats did not convene the Senate until 5 p.m. so their two presidential candidates — Senators Click here of New York and Click here of Illinois — could return from the campaign trail. Both spoke in favor of the legislation.

The campaign arm of Senate Democrats quickly issued news releases attacking Republicans up for re-election who opposed the procedural vote. And Democrats warned Republicans that they would hear from voters on an issue of major import to women as they accused Republicans of missing an opportunity to help victims of pay discrimination pursue justice in court.

“I have a terrible feeling the Senate just won’t get it,” said Senator Click here, Democrat of Maryland, who suggested that the still-male-dominated Senate remained out of touch about the nation’s changing workplaces. “But the women will get it, and we will start a revolution.”

Republicans said the proposal to ease the time constraints would prompt more lawsuits and lead to litigation over outdated cases. “This debate today is not about allowing, favoring or supporting discrimination,” said Senator Click here, Republican of Georgia.

Other Republicans said Democrats were simply determined to force potentially vulnerable senators up for re-election to take a tough vote while putting Senator Click here of Arizona, the presumptive Republican presidential nominee, on the spot.

“Welcome to gotcha politics 2008,” said Senator Mike Enzi, Republican of Wyoming.

Mr. McCain, who was campaigning in Louisiana, skipped the vote but told reporters he would have opposed the bill since it could contribute to frivolous lawsuits harmful to businesses.

Senator Click here, Republican of Utah, accused Democrats of unfairly trying to paint opponents of the bill as unsympathetic to victims of salary discrimination. “The only ones who will see an increase in pay are some of the trial lawyers who bring the cases,” he said.

Ms. Ledbetter, who is now retired, watched the debate from the Senate gallery.

Backers of the bill said it was unfair for the court to throw out a jury award in her favor because she had not initiated legal action within six months of the first instance of discrimination. They noted that she, like many others in the work force, could not be certain what her colleagues were making. Republicans said Ms. Ledbetter had testified during court proceedings that she was aware of some inequity in salary relative to male co-workers.

Ms. Ledbetter said the disparity spilled over into her pension and her Social Security payment. “They treated me like a second-class citizen not only when I worked, but for the rest of my life,” she said before the vote.

Six Republicans sided with Democrats, including four facing potentially difficult re-election fights: Senators Click here of Minnesota, Click here of Maine, Click here of Oregon and Click here of New Hampshire.

“We won’t take no for an answer,” said Senator Click here, Democrat of Massachusetts and a chief sponsor of the bill. “This issue isn’t going away.”