Sen. Bill Cassidy, R-La., says fixing Social Security is the top item on his legislative agenda before leaving office next year.

In an exclusive interview with CNBC published Tuesday, Cassidy acknowledged his less than cordial relationship with President Donald Trump, who endorsed Rep. Julia Letlow, R-La., over him and recently called him a "disloyal disaster." Cassidy said, though, his focus remains on addressing Social Security's looming funding crisis.

Cassidy lost his primary earlier this month and had a strained relationship with the president ever since be chose to be one of seven Republican senators to vote to convict Trump following the Jan. 6 Capitol protests.

"The longer you wait, the harder it is to fix, the more painful to fix," Cassidy said. "We need to do something now."

The urgency comes after Social Security trustees reported that the program's retirement trust fund could be exhausted by 2032, triggering automatic benefit cuts if Congress fails to act.

Cassidy is backing a bipartisan proposal that would invest $1.5 trillion in a separate market-based fund designed to help close much of Social Security's long-term funding gap without immediately raising taxes or cutting benefits.

While some analysts have questioned the plan's reliance on borrowing and future investment returns, Cassidy argues the concept is modeled after successful reforms to the federal Railroad Retirement system.

"All risk is borne by the fund; people would get their promised benefits," said Cassidy.

With his Senate term ending in January 2027, Cassidy said his status as a departing lawmaker gives him added motivation to tackle one of Washington's most politically difficult issues.

"I'm a retiring, so to speak, senator," Cassidy said. "I want to get it done before we leave, so there is impetus to get it done."

Cassidy is working with a bipartisan group of senators, including Democrat Sen. Tim Kaine, D-Va., and Republican Sen. Thom Tillis, R-N.C., to advance reform proposals.

He said the next steps include additional hearings and drafting legislative text while also identifying lawmakers willing to continue the effort after he leaves office.

"If it doesn't pass this Congress, I am speaking to colleagues who will be here next Congress and seeing who's interested in kind of carrying the torch," Cassidy said.

Cassidy's defeat highlighted Trump's continued dominance within Republican primary politics. Cassidy's vote to convict Trump in 2021 and subsequent breaks with the president left him politically isolated in a state where Trump remains overwhelmingly popular.

Still, Cassidy argued lawmakers should be willing to confront difficult policy challenges regardless of political consequences.

"We need to put the politics aside for the good of the country, for at least a little bit," Cassidy said.

"That's a challenge before members of Congress right now," he added.