HOUSTON  For all the support that the presidential candidates are expressing for renewable energy, alternative energies like wind and solar are facing big new challenges because of the credit freeze and the plunge in oil and natural gas prices.

Shares of alternative energy companies have fallen even more sharply than the rest of the stock market in recent months. The struggles of financial institutions are raising fears that investment capital for big renewable energy projects is likely to get tighter.

Advocates are concerned that if the prices for oil and gas keep falling, the incentive for utilities and consumers to buy expensive renewable energy will shrink. That is what happened in the 1980s when a decade of advances for alternative energy collapsed amid falling prices for conventional fuels.

“Everyone is in shock about what the new world is going to be,” said V. John White, executive director of the Center for Energy Efficiency and Renewable Technology, a California advocacy group. “Surely, renewable energy projects and new technologies are at risk because of their capital intensity.”