GM year-on-year sales fell 41% in November
Democrat Christopher Dodd said that letting Chrysler, General Motors or Ford fail would be playing "Russian roulette with the entire economy".
Failure would affect "almost every sector of the economy", he said.
He also severely criticised the US Treasury and financial institutions for their handling of the credit crisis.
"This is not about acting to save individual companies. If it were, I would let them fail," said Sen Dodd, who chairs the Senate Banking Committee.
With "hundreds of billions in outstanding debt obligations", he said the credit crisis would get a lot worse if the carmakers were not able to repay their debts.
Inaction, he said, "is simply not a solution."
Rounding off his opening salvo, the senator said the US was mired in a deep recession caused by "irresponsible actions in the financial sector", before criticising the Treasury Department for "misusing" its authority by spending the $700bn bail-out package agreed by Congress in an "ad hoc manner".
The authorities had not, he said, attached stringent enough conditions to the package.
Loan proposals
Earlier this week, Chrysler, Ford and General Motors submitted their proposals to Congress for multi-billion-dollar loans upon which their survival could depend.
The so-called Detroit Three of troubled US carmakers have asked for a combined total of $34bn (£22.8bn; 26.8bn euros).
Slashing costs, reducing levels of debt and investing in greener technologies form the centre-piece of each proposal.
The chief executives of Ford and GM have even offered to work for $1 a year if Congress approves the emergency aid.
General Motors asked Congress for a loan of $12bn, with an additional $6bn if necessary, to help it survive.
Ford requested a $9bn bridging loan, which it hopes it will not need.
Chrysler sought $7bn to survive the dramatic slump in sales that has decimated its cash reserves.
Avoiding criticism
The heads of all three companies decided not to use private jets to travel to Washington for their presentations to avoid public criticism.
GM chief executive Rick Wagoner and his counterpart at Ford, Alan Mulally, drove to Detroit in hybrid cars produced by their own respective firms.
GM has warned it could run out of cash in a matter of weeks and cannot wait until President-elect Barack Obama - who may be more sympathetic to industry pleas - takes office in in January.
But Republican critics and some Democrats say the financial crisis is not the only reason why the biggest US carmakers are in trouble.
They say that Ford, GM and Chrysler's production is inefficient, and that their labour costs are higher than many of their foreign rivals.
The Bush administration has offered to accelerate the payment of some $25bn in green investment credits already allocated to the car industry, but this has been opposed by Democrats in the House of Representatives.