Senator Amy Klobuchar, D-Minn., said she would have voted against the $25 billion auto industry bailout if it had come to a vote.
Congressional leaders delayed action on the bill after it became clear there were not enough votes for the package.
Klobuchar said she was initially inclined to support the rescue package, but changed her mind after listening to testimony this week from executives of the big three American auto makers.
Klobuchar said she wants the auto companies to present a plan showing the money will make them more financially viable.
"After listening to their testimony I think a lot of questions have been raised about the financial standing of the companies going forward. And I'm very concerned that the current packages planned would be nothing more than a temporary fix," Klobuchar said.
Senator Norm Coleman, R-Minn., came out against the plan earlier this week.
The auto executives warned that their industry could collapse without the loans.