WASHINGTON — The Treasury Department sank billions into auto finance giant GMAC without an exit strategy or proof the company was viable — a decision that could cost taxpayers $6.3 billion, a new watchdog report says.

The government said the $17.2 billion bailout was a necessary step to save troubled automakers General Motors and Chrysler. GMAC provides critical financing to auto dealers, who borrow to finance their fleets until the cars can be sold to consumers.

Yet GMAC faced far fewer conditions than the bailed-out automakers, the report says. When the automakers were rescued, they were forced into bankruptcy. Shareholders lost their investments, creditors took a hit and executives were forced to detail plans for making the companies viable.

GMAC was treated more like banks that received bailouts without having to explain what they were doing with the money, the report says.

The report was released today by the Congressional Oversight Panel overseeing the $700 billion financial bailout Congress passed in October 2008.

"Treasury missed many opportunities to improve accountability and protect taxpayer money," panel chair Elizabeth Warren said in a conference call with reporters.

She said Treasury didn't make GMAC show how it would return the taxpayer money, or how the investment would increase credit to consumers.

"These decisions mean that Treasury is now struggling to deal with a GMAC that is not financially rehabilitated, Treasury has no exit strategy and taxpayers are not fully protected," Warren said.

The Treasury Department responded by reiterating backing GMAC was necessary to preserve dealer financing for GM. It disputed the report's core finding, that alternative approaches might have saved taxpayer money and provided better transparency.