WASHINGTON  The homeowner assistance plan that Senator John McCain announced without detail in the presidential debate Tuesday night would allow millions of financially stretched Americans to refinance their mortgages with government help, but it would leave taxpayers to cover the losses, rather than the financial institutions that hold the original mortgages.

Mr. McCain said in the debate that the program would be expensive, and on Wednesday his chief economic adviser, Douglas Holtz-Eakin, acknowledged that the liability would be borne directly by taxpayers.

Mr. McCain, who has made cutting government spending a centerpiece of his campaign, justified the costs on Tuesday night by saying that “until we stabilize home values in America, we’re never going to start turning around and creating jobs and fixing our economy.”

As Mr. Holtz-Eakin described it, the proposal would ensure that homeowners got direct and speedier help to stay in their residences before they faced foreclosure, so that Wall Street and financial institutions would not be the only direct beneficiaries of government aid. The new loans would be 30-year mortgages at fixed interest rates of just over 5 percent, he said. The McCain campaign did not make clear what level of financial strain an applicant would have to show to qualify for the program.