WASHINGTON  The heads of the Big Three automakers of Detroit pleaded on Tuesday for emergency government aid to stave off potential collapse, but after four hours of testimony, it appeared they had not persuaded enough lawmakers to move quickly on a bailout.

Senate Democratic leaders said they had not been able to muster the support for legislation that would provide $25 billion to the troubled auto industry from the Treasury Department’s $700 billion economic rescue fund.

There is still a possibility that money may be freed up for Detroit from a previously approved loan program to help automakers retool their plants for more fuel-efficient vehicles.

But the industry hardly received a warm reception in Washington, despite its mounting troubles. The frantic bid from Detroit for help was laid bare at a packed hearing of the Senate banking committee, in which two of the three automakers said they might run out of money by the end of the year.