Sen. Bill Cassidy, R-La., is renewing his push for a major overhaul of Social Security, arguing Congress must act now to prevent future benefit cuts while avoiding payroll tax increases.

Cassidy's proposal would establish a separate $1.5 trillion investment fund designed to generate higher long-term returns than the Treasury securities held by Social Security's trust funds. The concept is modeled in part on the National Railroad Retirement Investment Trust, which invests pension assets in private markets.

Speaking to CNBC, Cassidy said the fund would be built gradually and remain separate from Social Security's existing trust funds. He said the goal is to strengthen the program's finances without reducing benefits for current retirees or increasing payroll taxes.

The renewed push comes after Social Security's latest trustees report projected the retirement trust fund will be able to pay full scheduled benefits only through 2033. Without congressional action, incoming payroll tax revenue would cover about 77% of scheduled benefits after that.

Cassidy has argued that delaying action will only make the problem harder to solve.

"The longer Congress does nothing, the larger the tax increase workers will face and the deeper the benefit reductions retirees will endure," Cassidy wrote this week on X. "I have a plan to save Social Security. Congress cannot wait any longer. Let's get it done."

Cassidy has worked on the proposal with Sen. Tim Kaine, D-Va., saying the investment fund could help close much of Social Security's long-term funding gap through stronger market returns.

The proposal faces significant political hurdles. Some lawmakers favor strengthening Social Security through higher payroll taxes, while others have proposed changes to eligibility or benefits.

Some policy analysts also have questioned whether borrowing money to invest in financial markets would expose taxpayers to additional risk, even if the strategy is intended to generate higher long-term returns.

Because major Social Security legislation typically requires bipartisan support to clear the Senate, Cassidy has urged lawmakers in both parties to begin negotiations before the program's financial outlook worsens.