A battle looms in Congress over how to pay for President Obama's pledge to restore the Gulf of Mexico to a condition better than before the Deepwater Horizon rig exploded and set off the largest oil spill in U.S. history.

Complete restoration of the Gulf Coast, battered by not only the spill but nearly a century's worth of oil and gas production, Mississippi River diversions and fierce hurricanes, could easily become the largest environmental restoration project in the world. Costs could run as high as $100 billion -- 10 times the cost of the Everglades restoration effort. Construction timelines would be measured in decades, rather than years.

But that multibillion-dollar price tag is expected to meet stiff resistance on Capitol Hill, amid high anxiety about the level of federal spending.

Environmentalists and advocates for the Gulf Coast region worry that as oil recedes from the marshes and the ocean's surface, taking the spill-related headlines with it, political will to devote the money needed for a massive environmental restoration effort will also dissipate.

"To be frank, it is going to be hard to convince Congress to give up those monies," said Aaron Viles, campaign director for the Gulf Restoration Network.

Unless Obama invokes emergency spending powers like those used by President George W. Bush to finance the wars in Iraq and Afghanistan, lawmakers will have to offset the cost through some combination of spending cuts or tax increases, according to aides and attorneys involved. The Congressional Budget Office will likely assign a price tag, or "score," to restoration legislation.

"The fight's going to be in the scoring of this," said one aide. "We need the administration to stand firm."

Further complicating efforts to divert most of that money to the region are arguments over how the money should be divided among Gulf Coast states.

"We certainly weren't equally hit by the oil," Louisiana Republican Sen. David Vitter said. "So I'll be fighting to ensure that the money follows the environmental impacts, which would push the great majority to Louisiana."

To pay for the restoration, the Obama administration wants Congress to devote a "significant amount" of the penalties companies responsible for the spill are expected to pay as a result of Clean Water Act litigation, a sum that could top $20 billion.

Under current law, whatever billions result from successful prosecution or settlement with BP PLC and possibly other companies will flow directly into a congressional trust fund created to pay oil spill cleanup costs. Lawmakers would then appropriate money from the fund to pay for environmental cleanup.

The 80 percent solution

Clean Water Act penalties stemming from the recent spill could reach $1,100 per barrel of spilled oil. If BP and the companies responsible are found to have committed gross negligence, the fines could rise to $4,300 per barrel. This means total penalties for the spill could range from $5.4 billion to $21.1 billion.

Louisiana lawmakers in the House and Senate have filed legislation to devote 80 percent of the spill penalties to restoring sediment-starved and polluted wetlands. The wetlands act as essential nurseries for the region's prized fish and shellfish and buffers against storm surge and hurricanes but continue to erode at the rate of a football field every 45 minutes.

"We do have the resources now available to help," Louisiana Democratic Sen. Mary Landrieu told the Oil Spill Commission late last month. "Not only fix what was damaged, stop the land loss, but invest smartly in the future of not just Louisiana and Mississippi, but the entire Gulf Coast, a region that is extremely important to our nation."

But the sponsors of the 80-percent proposals -- Vitter, Landrieu and Reps. Charlie Melancon (D) and Steve Scalise -- have not won any supporters outside the state's delegation. Landrieu, who continues to block President Obama's pick to fill the White House budget director vacancy over her concerns the administration is slow-walking drilling permits in the Gulf, began shopping her own 80-percent proposal to Gulf state senators in August but has failed to garner any signatures.

Some environmentalists say the administration seems more focused on midterm elections than on promoting Gulf restoration. They expect more action after Nov. 2.

Capturing 80 percent of those funds is "going to require leadership from Congress and certainly, leadership from the administration," said Dan Favre, communications director for the Gulf Restoration Network. "It remains to be seen whether that leadership will be present."

EPA Administrator Lisa Jackson, Obama's choice to lead his newly created Gulf restoration task force, "has begun discussing the work of the Task Force with a wide range of interested parties, including stakeholders, local officials and members of Congress -- particularly members from Gulf Coast states," according to spokesman Brendan Gilfillan.

Oil spill tax boost

Lawmakers also have recently proposed raising the tax flowing into the Oil Spill Liability Trust Fund dramatically, which aides say represents the likeliest means to pay for legislation that would divert money to the Gulf Coast.

Senate Majority Leader Harry Reid (D-Nev.) floated an oil spill-response bill this summer (S. 3663) that would have raised the tax to 49 cents per barrel. A separate package of tax cut extensions introduced by Senate Finance Chairman Max Baucus (D-Mont.) calls for hiking the tax to 78 cents. In May, the House passed a bill (H.R. 4213) that would raise the tax to 34 cents.

Oil industry advocates argue that increasing the per-barrel trust fund tax would be counterproductive because it would place an additional burden -- so soon after the recently lifted moratorium on Gulf oil drilling -- on an industry that lies at the center of the region's economy.

"Our general view is you can get more revenue out of the industry by allowing us to get back to work," said Martin Durbin, executive vice president for government affairs at the American Petroleum Institute. "Adding an additional tax on the industry is likely to have just the opposite effect."

Most agree the law establishing the trust fund, which has been mostly used to help pay the cleanup costs of much smaller oil spills, never contemplated a spill of the Deepwater Horizon's magnitude.

"This spill is the first big test of the Oil Pollution Act of 1990," said Michael LeVine, Pacific senior counsel for the environmental group Oceana. "The law has been implemented in smaller instances, never like this."

The fund was created in 1986 but not funded until the Oil Pollution Act of 1990, in the wake of the Exxon Valdez tanker spill. The 1990 law set a per-barrel tax that Congress elected to raise in 2008 from 5 cents to 8 cents per barrel, adding about a tenth of 1 percent to the cost of oil and building a current balance of $1.6 billion.

The Reid bill calls for raising the $1 billion cap on how much can be spent on cleanup of a single spill to $5 billion. In the case of the Deepwater Horizon incident, more than $630 million has been spent from the fund, according to Craig Bennett, director of the U.S. Coast Guard's National Pollution Funds Center, which oversees it.

"We're not in immediate threat of hitting that cap, but it's pretty easy to see that we could spend over $1 billion before this is said and done," Bennett said.