WASHINGTON – Humbled U.S. automakers pleaded with Congress Thursday for an expanded $34 billion rescue package, but heard fresh skepticism in a bumpy encore appearance.

“We made mistakes, which we’re learning from,” General Motors chief executive Rick Wagoner told the Senate Banking Committee.

Ford CEO Alan Mulally also acknowledged big mistakes, saying his company’s mantra once was “You build it, they will come.”

“We produced more vehicles than our customers wanted, then slashed prices,” he said. But as a result of these past mistakes, “we are really focused,” he said.

The Big Three executives made the trip from Detroit in new-model hybrid autos made by their respective companies, two weeks after a botched appeal for $25 billion in which they were chided for flying on private jets to beg for money.

Chrysler CEO Bob Nardelli promised that his company, recipient of a previous government-subsidized rescue loan in the 1970s that it repaid, would repay taxpayers by 2012 and would devote itself to manufacturing “fuel-efficient cars and trucks that people want to buy.”

Sen. Richard Shelby of Alabama, the senior Republican on the panel, complained that the pricetag on the package had jumped since the trio last appeared just two weeks ago

He pressed the automakers to explain why, and to justify how such aid would not simply “prop up a failed business model for a few months … and how are you going to pay it back to the taxpayers?”

Banking Committee Chairman Chris Dodd, D-Conn., who supports helping the industry, said detailed plans submitted earlier this week by the three auto companies on how they would use low-cost federal loans to reorganize still left a lot of questions unanswered.

Still, Dodd said, the economic news has become even more bleak since the auto executives appeared before Congress in late November.

“In just two weeks time, the clouds on the economic horizon have grown even darker and greater in number,” Dodd said, noting that the U.S. economy has now been declared official in a recession that began a full year ago.

Dodd said doing nothing and allowing one or all three of the companies fail, as some lawmakers have advocated, “plays Russian roulette with the entire economy of the United States.”

Several lawmakers in both parties, including Dodd, have pressed the automakers in recent days to consider a so-called “pre-packaged” bankruptcy in which they would negotiate with creditors in advance and downsize, then file for Chapter 11 protection in hopes of emerging quickly as stronger companies. The Big Three have publicly shunned the notion, but executives have indicated in recent days that it might ultimately be necessary.

GM’s survival plan envisions an administration-led restructuring overseen by a government oversight board.

“If more extensive restructuring is required, GM will work with the oversight board to determine the additional necessary actions,” GM’s written plan said.

United Auto Workers union president Ron Gettelfinger told the committee, “We are prepared to do our part.” But he also said workers for the auto companies shouldn’t have to make disproportionate sacrifices.

He also said that any kind of bankruptcy, even a pre-packaged one, was not “a viable option.” Gettelfinger claimed consuers would not buy autos from bankrupt companies, no matter the terms of the bankruptcy.

Congressional Democrats have urged the administration to tap into an already enacted $700 billion financial bailout program to help the auto industry.