Venture capitalist Nancy Floyd never saw lobbying as part of her job. But the founder of a California-based clean-tech investment firm has joined a gaggle of other solar energy investors on a lobbying pilgrimage to Capitol Hill.

Their message to lawmakers: Commit long-term funding to the nation’s renewable energy market — and quickly. Their investments are depending on it.

“I never thought as an investor I’d be as engaged in policy as I am — this is not in our job description,” said Floyd, managing director of Nth Power. “But I’m doing this because it’s so important. I can’t think of a bigger economic development opportunity facing this country, and we’re throwing ice water at it.”

Solar power is the fastest-growing energy source in the world. But in the United States, solar companies are seeing stocks slide, and major industry investors are threatening to withdraw if Congress doesn’t quickly extend the tax credits for renewables that are set to expire at the end of the year. 

The House voted last month to reroute $18 billion in oil company tax breaks to the renewable sector. But the move has energy advocates caught between powerful Washington lobby Big Oil and President Bush, who has already threatened a veto.

The oil and natural gas industry, with its strong GOP following, has already contributed more than $10.4 million to candidates this year, 72 percent of it to Republicans, according to the Center for Responsive Politics.

Even with oil prices soaring over $100 a barrel, oil lobbyists are preparing to fight hard for their credits, which they say will hold down gasoline prices and keep the major oil companies on a level playing field with other industries.

In comparison, the alternative energy sector has contributed nearly $500,000 to candidates so far this year, 71 percent of it to Democrats.

It’s a brewing fight that lobbyists from both industries are scrambling to avoid.

A number of solar supporters are already urging Congress to uncouple the renewable tax credits from Big Oil’s credits. Many oil companies are widely supportive of financing renewable development, with energy giants BP and Shell Oil Co. already investing billions of dollars to develop their solar businesses.

“We’re not against [the oil companies], and we didn’t propose this,” said the president of the American Council on Renewable Energy, Michael Eckhart. “We’re not out there to penalize anyone. But if tax credits are not extended, we will lose the following of Wall Street. It will be a national embarrassment.”

“At $100-something a barrel, we’re not requesting tax breaks to run our business,” said Shell President John Hofmeister. “However, we are deeply troubled that our business would be targeted by name for discriminatory tax treatment.”

As the bill heads to the Senate, both oil lobbyists and the National Taxpayers Union say they are focused on educating lawmakers, especially Democrats and fence-sitting moderate Republicans in oil-rich states, and warning that higher taxes would result in higher gasoline prices for consumers.

“Most oil companies are weighing the relative good versus harm,” said American Petroleum Institute tax policy analyst Mark Kibbe. “In the end, the harm is pretty significant. We shouldn’t discriminate against a form of energy that we’re still relying on.”

Solar investors maintain that the market is still strong, but many are finding strength in numbers through the National Venture Capital Association and other alliances. The Ceres coalition, for instance, brings together investors and major companies such as General Motors and Time Warner.

Time is of the essence for even the most successful solar businesses.

SunPower Corp., the leading solar power provider in the United States, recently lost 60 percent of its market cap over a three-week period — a $6 billion loss that the company blames on investors getting cold feet. The company, which provides solar systems for Macy’s, Wal-Mart and other major clients, is already limiting the number of facilities it can outfit before the tax credits expire.

“We’ve been faced with real uncertainty in the U.S. market,” said SunPower’s vice president of government affairs, Julie Blunden. “We’ve been fielding questions from Wall Street since the beginning of the year. We need to find a way to get this through in our own right, not lumped onto an oil industry vehicle.”

Last year, only 40 percent of SunPower’s revenue came from U.S. markets. And without the assurance of government funding, the company predicts domestic revenue could dip as low as 10 percent this year.

At the Washington International Renewable Energy Conference last week, the vice chairman of Credit Suisse, John Cavalier, warned the lender may stop funding projects starting in May if the incentives aren’t approved.

“Lenders rely on tax equity. Loans don’t work without tax credits. We can’t make loans to projects without tax credits,” he said. “You will begin to see lenders exit this business.”

Economists warn that if investors pull out, companies will begin sending jobs overseas to Germany and Spain, among other countries, where there is more government support.

Nth Power’s Floyd said she’s already eyeing overseas markets for expansion. The investment firm currently funds 42 companies in the U.S. and plans to add another 10 to 15 to its portfolio.

The National Petrochemical and Refiners Association, which advocates for many major oil companies, is using the same message, arguing that penalizing oil companies will stunt domestic development and cause increased dependence on foreign oil as companies move their projects overseas.

“This is not going to make us more energy independent,” said NPRA President Charles Drevna. “What we’re going to have is less energy security.”

But some solar giants say a battle between the two energy forces is inevitable. Barry Cinnamon, CEO of Akeena, one of the nation’s largest solar power installers, said he’s regularly on Capitol Hill, not only lobbying lawmakers but urging colleagues not to be intimidated by the major oil companies.

“Even if it means David versus Goliath, we have to engage in this battle,” he said. “Some people say the oil companies are too big, too nasty and have too much money. I understand that. But I don’t think it’s the right policy to keep giving incentives to people who don’t need it.”