WASHINGTON—Legislation has been introduced that would overturn the U.S. Supreme Court's 2009 ruling in Jack Gross vs. FBL Financial Services Inc., which made it easier for employers to defend age discrimination lawsuits, but the bill is given little chance of passage, observers say.

The Supreme Court held in its 5-4 ruling that plaintiffs in age discrimination cases must prove age was the determinative factor in an adverse job action, not just one of several motivating factors, to successfully pursue their case.

In sex and other discrimination cases, plaintiffs have to only establish that discrimination was a motivating factor in an adverse employment decision, and it does not necessarily have to be the determinative factor.

The proposed legislation, S. 2189, Protecting Older Workers Against Discrimination Act, which was introduced last month, would establish this same standard in age discrimination cases.

The bill states that Congress had intended that courts would interpret federal statutes, such as the Age Discrimination in Employment Act, that are similar to Title VII of the Civil Rights Act of 1964 “in ways that were consistent with the ways in which courts had interpreted similar provisions” in Title VII. The Gross decision “departed from this intent and circumvented well-established precedents.”

The bill has been referred to the Senate Committee on Health, Education, Labor and Pensions.

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A similar bill introduced in 2009 died in committee.

Observers say one positive factor that enhances the bill's chances of success is its bipartisan support. Its sponsors include Sens. Tom Harkin, D-Iowa, Patrick Leahy, D-Vt., and Chuck Grassley, R-Iowa.

Still, it is given little chance of success in today's congressional environment. Its prospects are “fairly slim,” said Todd A. Hanchett, a partner with law firm Stoel Rives L.L.P. in Portland, Ore.

The previous version did not even come up for a vote, and given the changes since in the House and Senate, one has to assume “it's fairly unlikely” this bill will be more successful, Mr. Hanchett said.