On average, women lose an estimated $700,000 over their lifetimes due to unequal pay practices, and this inequality means real hardships for their families.

And, while many factors influence a worker's earnings -- including educational attainment, work experience, and family status -- even when controlling for many of these variables, a substantial portion of the wage gap cannot be explained by anything but discrimination.

One of us experienced this discrimination personally. Over nearly two decades of work, Lilly received performance awards and outstanding reviews. Yet, late in her career, she learned that she had been paid significantly less than men in the company doing the exact same job. When she sued, a jury reviewed the evidence and concluded that she was paid less because of her sex.

Outrageously, the Supreme Court reversed the jury's verdict. They held that even though companies that discriminate do so covertly and do not reveal what male workers earn, Lilly somehow should have known that she had been discriminated against within 180 days of when she was hired. Because workers like Lilly do not learn of pay inequities for years, the decision left no recourse for her, or other victims of wage discrimination.

Largely because of Lilly's determination, the first legislation passed by Congress and signed into law by President Obama was the Lilly Ledbetter Fair Pay Act. Very simply, this law reversed the Court's decision.

We celebrate enactment of this important law, but we must recognize that it was only a first step. We need to do much more.

First, there are too many loopholes and too many barriers to effective enforcement of existing laws. That is why we strongly support the Paycheck Fairness Act. This bill -- which is sponsored by Senator Dodd, Senator Mikulski, and Representative Rose DeLauro -- would strengthen penalties for discrimination and give women the tools they need to identify and confront unfair treatment.

In January, the House of Representatives passed the bill overwhelmingly on a bipartisan basis. And, last month, the Senate Health, Education, Labor and Pensions Committee, chaired by Senator Harkin, held a hearing on this long-overdue bill. We hope that the Senate can pass the bill and send it to the President's desk this year.

In addition, we must recognize that the problem of unequal pay goes beyond insidious discrimination. As a nation, we unjustly devalue jobs traditionally performed by women, even when they require comparable skills to jobs traditionally performed by men. Why is a housekeeper worth less than a janitor? Why is a parking meter reader worth less than an electrical meter reader? To address this more subtle discrimination, Senator Harkin, along with Representative Norton, has introduced the Fair Pay Act to ensure that employers provide equal pay for jobs that are equivalent in skill, effort, responsibility and working conditions. We must act this year to pass this legislation, and eliminate the subtle and systematic issues that lead to unequal pay.

On this Equal Pay Day, let us recommit to eliminating discrimination in the workplace and ensuring that all Americans receive equal pay for equal work. America's working women and the families that rely on them deserve fairness on the job. Our aim is simple: We pledge to fight pay discrimination until we have achieved true equality in the workplace and there is no need to commemorate equal pay day any more.

Senator Tom HarkinThomas (Tom) Richard HarkinDem Senator open to bid from the left in 2020 Senate GOP rejects Trump’s call to go big on gun legislation Trump should require federal contractors to follow the law MORE (D-Iowa) is Chairman of the Senate Health, Education, Labor and Pensions Committee. Lilly Ledbetter was the plaintiff in the American employment discrimination case
Ledbetter v. Goodyear Tire & Rubber Co. The Lilly Ledbetter Fair Pay Act of 2009 is named after her.