Officials of the nation’s largest public transit agencies came to Capitol Hill Tuesday seeking legislation designed to save them from imminent default.

The transit agencies hope Congress will include a provision requiring the Treasury Department to guarantee billions of dollars in financing agreements at risk because of American International Group Inc.’s collapse in any economic stimulus bill voted on this week.

At issue are transit financing deals guaranteed by AIG from 1988 to 2003. Terms of the complicated financial deals required the insurer guaranteeing the transactions to maintain a AAA credit rating. The downgrading of AIG’s credit rating this year suddenly put the transit agencies into technical default on the financing, allowing investors to demand full payment and hefty termination fees.

The deals affect cities across the country from Los Angeles to Houston and St. Louis to Washington.

Beverly Scott, chairwoman of the American Public Transit Association and chief executive of Atlanta’s transit authority, said “31 of the nation’s largest transit systems would be financially crippled” if nothing is done.

“The innocent victims will be the millions of riders who rely on public transit everyday,” she added.

Michael Wiley, chief executive of Sacramento’s regional transit authority, said his agency would lose 20 percent of its operating budget if it is forced to make the payments on the financing deals.

The transit officials say its federal government encouraged the bank deals as a creative tool for financing infrastructure investments. As Congress designs a new surface transportation authorization next year with new funding streams, Scott said, lawmakers will have a hard time selling public-private partnerships if they allow default.

The agencies and some of their congressional supporters have asked Treasury Secretary Henry M. Paulson Jr. to step in, but so far there has been no decision.