(CNN) -- In this election year, when support from women is considered especially crucial, Democrats in Washington made a high-profile push Monday for a bill to help prevent pay discrimination against women.
"Congress has to step up and do its job," President Obama told supporters of the bill on a conference call.
"If Congress passes the Paycheck Fairness Act, women are going to have access to more tools to claim equal pay for equal work. If they don't, if Congress doesn't act, then women are still going to have difficulty enforcing and pressing for this basic principle."
Republicans, who argued the measure would lead to over-burdensome government involvement in private sector pay practices, are expected to block the bill when it comes up for a key procedural vote in the Senate Tuesday.
Publicly, Democrats touted the measure, saying it would make it easier for women to prove pay inequity and sue for damages. They said it would provide pay fairness to female workers and would boost the economy because so many women -- including many heads of households - are being underpaid. Privately, Democrats acknowledged they expect also to get a political boost from the legislative fight because GOP opposition could leave Democrats better positioned to appeal to female voters this fall.
"American women are mad as hell," said Sen. Barbara Mikulski of Maryland, the Democrats' chief sponsor of the bill. She said women earn 77 cents to the dollar for their male counterparts, which is up 18 cents since 1963, when President Johnson signed into law the Equal Pay Act.
In conference calls, press releases, and floor speeches, Democrats argued the pay inequality caused women to lose thousands of dollars over their lifetime and hurt poor women and minorities even more.
"No woman working to support herself and her family should be paid less that her male counterparts," said Senate Majority Leader Harry Reid, D-Nevada.
In a letter to Senate leaders, the U.S. Chamber of Commerce and other business groups said while they have "no tolerance for discrimination" they would "vigorously oppose" the bill.
"The Paycheck Fairness Act would impose unprecedented government control over how employees are paid at even the nation's smallest employers," the letter said. "This flawed legislation would outlaw many legitimate practices that employers currently use to set employee rates, even where there is no evidence of intentional discrimination."
The business groups said the bill might prevent employers from offering "premium pay for professional experience, education, shift differentials or hazardous work as well as pay differentials based on local labor market rates or an organization's profitability."
Mikulski said her bill would build on the 1963 law. Specifically, it would require employers to prove pay inequity is due to job performance, not gender, and would, for the first time, prevent employers from forbidding employees from sharing salary information with each other. The bill also would allow women to sue for back pay and punitive damages if they suffered pay discrimination.
Aides in both parties said they don't expect more than a handful of defections from both sides of the aisle Tuesday.
Sen. Scott Brown of Massachusetts is in a tough and closely watched re-election fight against a woman, Harvard law professor Elizabeth Warren. He is one of the Republican senators Democrats hoped to put in a difficult political position by pushing the pay legislation.
But Brown's office said Monday he will vote against the measure.
"Sen. Brown believes strongly in fair pay, and that employers who discriminate against women should be prosecuted aggressively," said Marcie Kinzel, his Senate spokesperson. "However, on the bill before the Senate, Sen. Brown believes it will put more burdens on small businesses and could lead to job losses at a time when our economy can least afford it."