There are or were tax rebates for purchase of hybrid cars, and tax deductions for large cars used in business, so the mechanics exist.
If it were for a short time, i.e. 6 months to a year, then while it may act the same as a tariff, it would be very short lived. The object to spur the economy, and get the consumer buying. Detroit said they are in this fix because of finance and people are not buying cars.
I for one, am against the Auto Bailout. I would let them file Chap 11 and re-organize. I don't buy all the hype about people not buying from a company in Chap 11. Continental and United Airlines both were in Chap 11, reorganized and Continental is one of the strongest today. Will it hurt, sure, would it be a disaster, I don't think so.
Detroit must build cars people want to buy, at a price that makes a profit. They are no different than any other business. If they can't they should not be in business. Ask the workers of Eastern, Pan Am, Braniff, Ozark and many other airlines. They could not get their cost in line and they are no more. The U.S. Government should not be in the business of business!