WASHINGTON — Negotiations on a homeowner rescue package that could help half a million strapped borrowers get government-backed mortgages hit a snag yesterday over how to pay for the plan.

Democrats were balking at a demand by Republicans to tap a fund designed to pay for building housing for poor families to finance the new foreclosure-prevention program. Without it, Republicans call the program a bailout that would open taxpayers to undue risk.

The dispute dimmed the prospects of a swift deal between Sen. Christopher J. Dodd, D-Conn., the Banking Committee Chairman, and Sen. Richard C. Shelby of Alabama, the panel's senior Republican. The two said yesterday that they were near agreement, but a committee session to vote on the measure — originally scheduled for yesterday morning — did not occur.

"It is my desire that we fashion legislation that will enjoy broad-based support. It may not be possible in the end," Dodd said before announcing the session would be delayed further while negotiations continued. "If it doesn't happen, so be it, but we're not going to have it not happen because we didn't try."

Even as Democrats and Republicans raised concerns about the outlines of the agreement, aides held out hope of a compromise.

"We believe we have an agreement in concept," said Jonathan Graffeo, Shelby's spokesman.

The package would tighten regulation of Fannie Mae and Freddie Mac, the government-sponsored companies that finance mortgages. It would also allow the Federal Housing Administration to back up to $300 billion in new loans for homeowners facing foreclosure, who would otherwise be considered too financially risky to get a fixed-rate, government-insured loan.

And under the tentative agreement with Republicans, a $600 million-per-year affordable housing fund financed by Fannie Mae and Freddie Mac would be partially diverted to cover the projected $1.7 billion cost of the FHA mortgage rescue plan.

But some Democrats are reluctant to go along with tapping money designed to help poor people afford housing to pay for the anti-foreclosure program.

"It's a big problem for Democrats, because we've all been advocating an affordable housing fund for years now," said a senior aide, speaking on condition of anonymity because talks on the measure were ongoing. "We don't want that money to go to the foreclosure (plan). We need money for both."

Affordable housing advocacy groups unleashed a barrage of telephone calls and e-mails on senators yesterday to protest the emerging agreement.

"We will be deeply disappointed if money which has been advocated for and intended for some time to help build housing for extremely low-income people gets diverted away for this foreclosure problem," said Sheila Crowley, president of the National Low Income Housing Coalition. "It would be just a travesty."

A major sticking point has been how to insulate taxpayers from risk should homeowners who got government help default on their new mortgages.

President Bush, who has threatened to veto a similar House-passed bill, cites exposing taxpayers to potential losses as a top concern. The White House calls the plan a burdensome bailout.

Under the proposed FHA program, borrowers would have to show they could afford the new loans, while mortgage holders would have to agree to take a substantial loss on the existing loan in exchange for avoiding a costly foreclosure. The FHA would share at least half of any proceeds if the homeowner refinanced again or profited from selling the home.