WASHINGTON — Humbled U.S. automakers pleaded with Congress Thursday to approve an expanded $34 billion rescue package, but heard fresh skepticism from lawmakers in a bumpy encore appearance.

“We’re here today because we made mistakes,” General Motors chief executive Rick Wagoner told the Senate Banking Committee in prepared testimony.

The three executives made the trip in new-model autos made by their respective companies, two weeks after a botched attempt for aid that included harsh criticism of corporate leaders who flew here on private jets to beg for money.

Ahead of testifying before the Senate Banking Committee, Wagoner apologized for asking for the help from taxpayers. Speaking with reporters, he said, “We wish the market conditions were better. They’re not.”

Chrysler CEO Bob Nardelli said: “I can tell you in my 38 years in business, I’ve never attended a more important session where more is reliant upon both the House and the Senate.”

Ford CEO Alan Mulally said in his prepared remarks that while his company isn’t in as desparate straits as rivals GM and Chrysler, his company could still use a federal guarantee of some $9 billion “as a critical backstop” in a stressed economy.

“Our plan is working, but there is clearly more to do — something that is increasingly difficult in this tough economic climate,” he said.