WASHINGTON — The Senate Judiciary Committee voted today to repeal a federal law defining marriage as a union between a man and a woman, but the legislation lacks votes for passage in either House.

The vote was 10-8, with all committee Democrats favoring appeal and all Republicans opposed. The only immediate effect is political: Democrats can show part of their liberal base of backers that they strongly support equality in federal benefits for gay couples.

The repeal bill would need 60 votes in the 100-member Senate, and sponsors acknowledged the votes aren’t there. The measure would have no chance in the House, controlled by conservative Republicans.

The current federal law, known as the Defense of Marriage Act, has a huge negative economic impact on gay couples through the denial of federal government benefits.

Those couples cannot file joint federal income tax returns and take deductions available in traditional marriages. There are no spousal Social Security benefits. They can’t take advantage of the Family and Medical Leave law that protects one’s job and health insurance during emergency absences. Surviving gay spouses have no protection from estate taxes.

Because of the law, “thousands of American families are now being treated unfairly by their federal government,” said the committee chairman, Sen. Patrick Leahy, D-Vt. “They are shunted aside — singled out from all other marriages recognized by their states.”

Sen. Charles Grassley of Iowa, the committee’s top Republican, called it “simply wrong to claim that the bill would create federal benefits for all lawfully married couples. In reality, it would create federal benefits for many same-sex couples who are not lawfully married.”

Grassley said he was referring to the repeal bill’s federal recognition of a same-sex marriage, even if the legally married couple moves to a state where gay marriage is illegal.

The bill’s chief sponsor, Sen. Dianne Feinstein, D-Calif., said the law “is discriminatory and should be stricken.” Numerous businesses, she said, have supported repeal because they need to maintain a separate set of books when calculating health and retirement benefits.

Sen. John Cornyn, R-Texas, said Senate Majority Leader Harry Reid, D-Nev., “has no intention of bringing this bill up this year or next. Reid “would face a revolution in his own caucus” if he did, Cornyn said.

He said the Democrats were trying to satisfy their gay marriage supporters for the 2012 election.

In advance of the vote, Leahy had defended the timing, saying that “it is never the wrong time to right an injustice.”

It is likely that the issue will be debated right up to the 2012 elections, while challenges to the law take place in several federal appeals courts. Conservatives pledged to make it a front-burner issue after President Barack Obama decided in February his administration no longer would defend the law.

Much has changed since President Bill Clinton signed the Defense of Marriage Act in 1996. The District of Columbia and six states now recognize gay marriage: Massachusetts, Connecticut, Iowa, Vermont, New York and New Hampshire.