President Barack Obama and Senate Democrats are courting female voters by pushing a bill that calls for equal pay in the workplace.
Democrats say women now earn on average 77 cents for every dollar earned by a man and that the bill would require companies to show that differences in wages are not based on gender.
It also would prohibit retaliation against employees who reveal their own wages or seek details about their employers' pay policies.
Republicans say the legislation would place unnecessary burdens on businesses.
The measure is not expected to win the 60 votes needed to advance Tuesday.
It is a device Democrats are using to try to protect their political advantage among critical women voters in this year's presidential and congressional elections.