Wind and solar power have been growing at a blistering pace in recent years, and that growth seemed likely to accelerate under the green-minded Obama administration. But because of the credit crisis and the broader economic downturn, the opposite is happening: installation of wind and solar power is plummeting.

Factories building parts for these industries have announced a wave of layoffs in recent weeks, and trade groups are projecting 30 to 50 percent declines this year in installation of new equipment, barring more help from the government.

Prices for turbines and solar panels, which soared when the boom began a few years ago, are falling. Communities that were patting themselves on the back just last year for attracting a wind or solar plant are now coping with cutbacks.

“I thought if there was any industry that was bulletproof, it was that industry,” said Rich Mattern, the mayor of West Fargo, N.D., where DMI Industries of Fargo operates a plant that makes towers for wind turbines. Though the flat Dakotas are among the best places in the world for wind farms, DMI recently announced a cut of about 20 percent of its work force because of falling sales.

Much of the problem stems from the credit crisis that has left Wall Street banks reeling. Once, as many as 18 big banks and financial institutions were willing to help finance installation of wind turbines and solar arrays, taking advantage of generous federal tax incentives. But with the banks in so much trouble, that number has dropped to four, according to Keith Martin, a tax and project finance specialist with the law firm Chadbourne & Parke.

Wind and solar developers have been left starved for capital. “It’s absolutely frozen,” said Craig Mataczynski, president of Renewable Energy Systems Americas, a wind developer. He projected his company would build just under half as much this year as it did last year.

The two industries are hopeful that President Obama’s economic stimulus package will help. But it will take time, and in the interim they are making plans for a dry spell.

Solar energy companies like OptiSolar, Ausra, Heliovolt and SunPower, once darlings of investors, have all had to lay off workers. So have a handful of companies that make wind turbine blades or towers in the Midwest, including Clipper Windpower, LM Glasfiber and DMI.

Some big wind developers, like NextEra Energy Resources and even the Texas billionaire T. Boone Pickens, a promoter of wind power, have cut back or delayed their wind farm plans.

Renewable energy sources like biomass, which involves making electricity from wood chips, and geothermal, which harnesses underground heat for power, have also been slowed by the financial crisis, but the effects have been more pronounced on once fast-growing wind and solar.

Because of their need for space to accommodate giant wind turbines, wind farms are especially reliant on bank financing for as much as 50 percent of a project’s costs. For example, JPMorgan Chase, which analysts say is the most active bank remaining in the renewable energy sector, has invested in 54 wind farms and one solar plant since 2003, according to John Eber, the firm’s managing director for energy investments.

In the solar industry, the ripple effects of the crisis extend all the way to the panels that homeowners put on their roofs. The price of solar panels has fallen by 25 percent in six months, according to Rhone Resch, president of the Solar Energy Industries Association, who said he expected a further drop of 10 percent by midsummer.

(For homeowners, however, the savings will not be as substantial, partly because panels account for only about 60 percent of total installation costs.)

After years when installers had to badger manufacturers to ensure they would receive enough panels, the situation has reversed. Bill Stewart, president of SolarCraft, a California installer, said that manufacturers were now calling to say, “Hey, do you need any product this month? Can I sell you a bit more?”

The turnaround reflects reduced demand for solar panels, and also an increase in supply of panels and of polysilicon, a crucial material in many panels.

On the wind side, turbines that once had to be ordered far in advance are suddenly becoming available.

“At least one vendor has said that they have equipment for delivery in 2009, where nine months ago they wouldn’t have been able to take new orders until 2011,” Mr. Mataczynski of Renewable Energy wrote in an e-mail message. As he has scaled back his company’s plans, he has been forced to cancel some orders for wind turbines, forfeiting the deposit.

Banks have invested in renewable energy, lured by the tax credits. But with banks tightly controlling their money and profits, the main task for the companies is to find new sources of investment capital.

Wind and solar companies have urged Congress to adopt measures that could help revive the market. But even if a favorable stimulus bill passes, nobody is predicting a swift recovery.

“Nothing Congress does in the stimulus bill can put the market back where it was in 2007 and 2008, before it was broken,” said Mr. Martin, the tax lawyer with Chadbourne & Parke. “But it can help at the margins.”

The solar and wind tax credits are structured slightly differently, but the House version of the stimulus bill would help both industries by providing more immediate tax incentives, alleviating some of their dependency on banks.

Both House and Senate would also extend an important tax credit for wind energy, called the production tax credit, for three years; previously the industry had complained of boom-and-bust cycles with the credit having to be renewed nearly every year.

Over the long term, with Mr. Obama focused on a concerted push toward greener energy, the industry remains optimistic.

“You drive across the countryside and there’s more and more wind farms going up,” said Mr. Mattern of West Fargo. “I still have big hopes.”