WASHINGTON  Why, after decades of battling, complaining and maneuvering over fuel economy standards, did carmakers fall in line behind the tough new nationwide mileage standard President Obama announced Tuesday?

Because they had no choice. The auto industry is flat on its back, with Chrysler in bankruptcy, General Motors close to it, and both companies taking billions of dollars in federal money. Foreign automakers are getting help from their own governments. Climate change legislation is barreling down the track, and Congress showed last fall that it had no appetite to side with Detroit any more.

Simply put, Detroit and the other companies need Washington’s help, and they are powerless to block the rules Washington dictates.

“They can feel the political winds changing,” said David Doniger, a lawyer with the Natural Resources Defense Council who has faced the car companies in court many times. “They need government aid to stay in business. When you have your hand out for help, it’s hard to use the same hand to thumb your nose at the federal government.”