DETROIT — U.S. consumers continued to clamp down on spending on new cars and trucks in March as their confidence in the economy dipped to its lowest level since the invasion of Iraq in March 2003.

They were more than assisted in their restraint by banks, which have started to make it increasingly difficult to get auto loans — especially for customers with marginal credit — as the nation simultaneously battles woes in the housing, credit and energy sectors.

Americans bought just 1.35 million new vehicles last month, a decline of 12 percent from a year ago. That translated into an annual selling rate of 15.1 million cars and trucks — an extremely soft kickoff for the spring selling season that left some wondering whether the economy had yet hit bottom or was still getting worse.

"I'd like to be able to tell you that the worst is behind us, but I really can't give you that assurance," Jim Farley, Ford's group vice president of marketing and communications, said in a conference call. "At this point, our sense is that the next quarter, the second quarter of the year, may be our most difficult of the year."

Mark LaNeve, vice president for General Motors Corp.'s North America vehicle sales, service and marketing, seemed a bit more upbeat but emphasized "nobody knows" where the bottom really is in this market.

That is especially true, he noted, considering all the uncertainties afoot. Policymakers continue debating financial changes that may or may not come to pass, and it is unclear how consumers will spend their $300-$1,200 rebate checks, part of the Bush administration's stimulus package.

"I think we have just as many reasons to be positive as to be negative," LaNeve said. "So I choose to be positive. And I really believe that we will begin recovering."

Every major automaker posted declines in March. Sales were down 18.7 percent at General Motors Corp., 14 percent at Ford Motor Co. and 19.4 percent at Chrysler Corp.

But Asian automakers, with a reputation for making more fuel-efficient cars, especially subcompacts and hybrids, fared much better. Sales were down a lesser 3.2 percent at Honda Motor Co. and 3.8 percent at Nissan Motor Co. Toyota Motor Corp. sales fell by double digits, 10.3 percent.

Despite the declines, neither GM nor Ford cut their production plans for the second quarter, saying they had anticipated much of the downturn.

LaNeve said he remained hopeful that demand for new cars and trucks continues to build up as consumers rein in their spending in uncertain times.

Eventually, he noted, confidence will be restored and millions of consumers who have been holding back will return to showrooms, even though it wasn't exactly clear how soon that might happen.

"I would call that 2 million units of pent-up demand that's sitting out there that's going to come into the market at some point," LaNeve said.

Mike DiGiovanni, GM's executive director of global markets and industry analysis, said he expects the market to gain traction as soon as stimulus efforts are given a little time to work.

"We'll start to see mortgage rates continue to fall, we'll start to see people starting to spend their stimulus checks, we'll see some of the housing imbalances looking a little bit better," he said. "We don't expect nirvana in the next 60 days, but we expect to see some positive signs."

If there were any messages to be gleaned from car sales, which were down 5.4 percent last month, and truck sales, which were off 17.8 percent, it was this: Consumers are looking for models with a lower cost of ownership.

That didn't just include cars that offer better fuel efficiency, but those that also came with a lower price tag, and in turn, a lower monthly payment, which might offset other increases in their household budgets.

"The consumers we see are really seeing a lot of pressure from housing prices, fuel prices and also the tightening of credit," said Steven Landry, Chrysler executive vice president for North American sales. "We see our consumers coming into our dealerships and wanting to trade down into a lower monthly payment."