In an op-ed piece in the St. Petersburg (Fla.) Times, McCain called the legislation, which President George Bush has said he would sign, a "sweetheart deal" that could potentially cost taxpayers hundreds of billions of dollars.

The measure, supported by Treasury Secretary Henry Paulson, would bail out Fannie Mae (NYSE:FNM) and Freddie Mac (NYSE:FRE), the two quasi-government lenders that guarantee some $5 trillion in mortgages -- about half of the nation's total mortgage bill.

"Fannie and Freddie are the poster children for a lack of transparency and accountability. … They've misled us about their accounting, and now they are endangering financial markets," McCain said.

McCain noted he called for mortgage market reform more than two years ago. He said Fannie Mae and Freddie Mac have grown too large to be allowed to sink, but, he said, if any taxpayer money winds up going to them, "the management and board should immediately be replaced, multimillion-dollar salaries should be cut, and bonuses and other compensation should be eliminated."

"Even with those terms, sticking Main Street Americans with Wall Street's bill is a shame on Washington," he said.