For years, environmentalists have successfully fought to maintain a Congressional ban that keeps large tracts of the Outer Continental Shelf off-limits to oil and gas production.

The ban has stood for more than two decades, even in recent years when pro-drilling Republicans controlled both Congress and the White House.

But $4-a-gallon gasoline has softened public sentiment toward opening up the potentially environmentally sensitive ocean territory.

And that has boxed environmental groups into a previously unthinkable position: scrambling to head off an 11th-hour legislative push to ease those long-standing restrictions — a move that is increasingly supported by their Democratic allies.

Former Speaker Newt Gingrich (R-Ga.) last month forecast that Democratic support would tip the scales in favor of lifting the drilling moratorium.

“My prediction is that by September the Democrats will have split into a pro-energy faction and an anti-energy faction, and the Republicans and the pro-energy Democrats will be an absolute majority in the House,” he told reporters in the Capitol last month after a news conference about House Republicans’ recess energy protest.

Indeed, dozens of House Democrats have signed on to a bipartisan bill sponsored by Reps. Neil Abercrombie (D-Hawaii) and John Peterson (R-Pa.) that would end the moratorium and invest billions of dollars of new lease revenues into alternative energy.

More than one-quarter of House Members are supporting the bill, with more co-sponsors expected when lawmakers return this week.

And in the Senate, five Democrats have joined with five Republicans to write a bill that would allow some states to opt out of the drilling ban, while also promoting renewables and other energy sources.

The “Gang of 10” has seen its ranks swell as well, with the addition of three Democrats and three Republicans over recess. More Senators are expected to join as the group introduces its bill, possibly as soon as this week.

Drilling opponents note that it will take as much as a decade for oil and gas from newly opened Outer Continental Shelf areas to be brought to market.

The U.S. government’s own Energy Information Administration last year said lifting the ban in the continental United States would produce about 200,000 more barrels of crude oil a day — in 20 years.

However, the amount is such a small portion of global oil use that the EIA deemed it would have an “insignificant” effect on gas prices.

But drilling proponents argue that opening up the Outer Continental Shelf shows the United States is serious about curbing its dependence on foreign energy sources.

The political dynamic is frustrating for environmentalists, who for the past two years have enjoyed newfound clout with the Democratic majority and seen their causes elevated to the top of the legislative agenda.

Environmentalists complain that while expensive gas may well have turned many consumers against the oil companies, it has also allowed the industry to make the case that the answer to higher prices is to increase its production capabilities, if necessary by drilling in the Outer Continental Shelf.

“The Big Oil companies took something that was a real disadvantage and somehow parlayed that into massive support for giving them more profits,” said Anna Aurilio, the Washington director for Environment America. “To me that is unbelievable.”

While both Speaker Nancy Pelosi (D-Calif.) and Senate Majority Leader Harry Reid (D-Nev.) have taken a dim view of drilling, a steady drumbeat from Republicans and moderate Democrats has forced both to reluctantly reconsider their drilling stances.

Pelosi previously called Republican calls for expanded drilling a “hoax” that would have little or no impact on gas prices, but the House Democratic leadership is currently drafting an energy bill that will, among other things, open new areas of the Outer Continental Shelf for “responsible” drilling.

The bill is expected to be brought to the floor this month.

And while Reid prefers policies to promote alternative energy sources, he, too, recently signaled a drilling vote was likely this month in the Senate, which deadlocked for weeks before the August recess over drilling.

Allowing drilling votes could prevent energy tension from spilling over to the continuing resolution, a must-pass bill that will fund the government after Sept. 30.

But the CR will also be the vehicle for extending the annual drilling moratorium, and Republicans have suggested they will hold up the bill, raising the specter of a government shutdown.

Environmentalists are preparing for the showdown by running ads and mobilizing grass-roots efforts in dozens of states in support of their anti-drilling, pro-renewable energy message.

“At the very basic level, one fact stands: Opening up drilling of any kind is going to do nothing to reduce the price of gas,” said Marchant Wentworth, a legislative representative of the Union of Concerned Scientists’ Clean Energy Program.

A coalition of national environmental groups, including the League of Conservation Voters, Environment America and the Sierra Club, last week also unveiled the “Paid for by Big Oil” campaign, which seeks to highlight oil company contributions to Republicans in dozens of states.

But Democratic drilling supporters note that political realities dictate that a production component be included in any comprehensive energy package — including bills with clean-energy provisions favored by environmentalists, such as incentives to develop wind and solar power.

“If we’re going to get something through the Congress, we’ve got to have drilling,” said Chris Thorne, a spokesman for Sen. Kent Conrad (D-N.D.), who helped launch the Gang of 10 effort.

But despite increasing bipartisan support for drilling, proposals to ease the offshore moratorium are likely to be coupled with provisions unpalatable to industry groups. Pelosi has said the bill she’s writing will include new federal renewable energy standards and the repeal of billions of dollars of tax breaks for oil and gas companies — measures strongly opposed by the industry and that have both repeatedly failed in the 110th Congress.

The bipartisan Senate proposal would also revoke billions of dollars in tax breaks, earning it the opposition of the American Petroleum Institute and other industry heavyweights.

“[I]mposing damaging new and permanent taxes on American businesses — our members — will neither contribute to our energy supply nor reduce consumer demand,” wrote the Coalition for Affordable American Energy, which includes the U.S. Chamber of Commerce and the National Association of Manufacturers, to Senators last week.

While many observers expect only continued gridlock over energy in the coming weeks, environmentalists say drilling talk has overshadowed what they call a “teachable moment” about the dangers of U.S. oil dependence.

“The Big Oil companies have sort of co-opted this moment to get exactly what they want, and you know, we’re going to fight like hell to make sure they don’t,” Environment America’s Aurilio said.