WASHINGTON (Reuters) - U.S. Senate Republicans on Wednesday blocked legislation to reverse a Supreme Court ruling that makes it tougher for workers to sue for pay discrimination.

Democratic presidential rivals Barack Obama and Hillary Clinton interrupted their campaigns to return to the Senate to vote for the bill. The measure would lift tight time restraints to file claims that could expire before workers realize they were treated unfairly.

On a 56-42 vote, mostly Democratic supporters of the bill fell short of the needed 60 in the 100-member Senate to clear a Republican procedural hurdle and move toward passage of the bill approved earlier by the House of Representatives.

The Senate action likely kills the bill for the year.

Yet it is certain to be an issue in this November’s congressional and presidential elections, particularly among female voters sensitive to pay inequity between the sexes.

The blocked Lilly Ledbetter Fair Pay Act, named for an Alabama woman who lost her case in the Supreme Court last year, is backed by women’s and civil rights groups that argue it would give workers a fair chance for justice.

It is opposed by the White House and business groups, which warn it would trigger an explosion of lawsuits and allow suits to be filed years and even decades after alleged offenses.

On average in the United States, women are paid about 23 percent less than men, while minorities receive even less -- despite laws that mandate equal pay for equal work.

In a failed plea for the bill before the vote, Clinton, who would be the first female U.S. president, told colleagues: “I’m hoping you will stand up and vote to make it clear that women who get up every single day and go to work deserve to be paid equally to their male counterparts. That’s all Lilly Ledbetter wanted.”

The White House said it opposed discrimination in the workplace. But it threatened to veto the bill if Congress passed it, saying in a statement the measure would “impede justice and undermine the important goal of having allegations of discrimination expeditiously resolved.”

Obama, who would become the first black U.S. president, said in a statement: “This pay gap is an ugly reflection of the discrimination that still exists in the workplace. And as the son of a single mother and the father of two young daughters, I believe we have a responsibility to close it.”

Backers of the bill complained that the Supreme Court, in its 5-4 ruling last May, reversed decades of legal precedent by declaring discrimination claims must be filed within 180 days of the first alleged offense.

The court rejected the U.S. Equal Employment Opportunity Commission’s contention that each new discriminatory paycheck triggers a new 180-day statute of limitations.

The failed legislation would have amended the 1964 Civil Rights Act by putting the old EEOC standard into law, and cover pay discrimination based on gender, race, national origin, religion, age and disabilities.

Senate Republican leader Mitch McConnell denounced the bill, saying, “The fundamental problem is that it creates massive new opportunities to sue.”

Sen. John McCain of Arizona, the Republican presidential candidate, did not return to Washington to vote on the bill.

Instead, he continued his tour of depressed areas of the United States. Poverty has been partly linked to the wage gap, particularly among households headed by single mothers.

(Additional reporting by Richard Cowan)