PENSACOLA, Fla. -Laying the groundwork for an evening speech to the nation, President Barack Obama walked a pristine stretch of sand on Florida's shoreline Tuesday and pledged to "fight back with everything we've got" against the spreading oil lurking offshore.

In a speech at Pensacola's Naval Air Station, Obama took note of the painful contrasts around him: "The sand is white. The water's blue," he said. And yet, he added, "those plumes of oil are off the coast."

Obama's challenge was spelled out clearly in a sign held up by one of the passersby who watched the president's motorcade whisk through this beach town: "Lead now!" it commanded.

That same sentiment was reflected in a new Associated Press-Gfk poll released Tuesday that found a majority of Americans disapprove of how Obama has handled the spill.

Speaking to troops at the base, Obama said the country faced an unprecedented environmental disaster and "we're going to continue to meet it with an unprecedented response."

"We're going to fight back with everything that we've got," he said.

With that, the president wrapped up a two-day visit to the Gulf and headed back to Washington to outline his plans for the Gulf in a prime-time speech from the Oval Office. One measure of the enormity of the problem: The oil that has gushed into the gulf would fill the Oval Office nearly 600 times over, based on the government's best estimate of how much has been spilling daily.

White House spokesman Robert Gibbs said earlier Tuesday that Obama was poised to seize the handling of oil spill damage claims from BP, if necessary, to ensure that people get the help they need to recover.

The president began his day by inspecting Gulf waters from the unsullied white sands of Pensacola Beach with Gov. Charlie Crist and Coast Guard Adm. Thad Allen. Not far away, people were swimming in the glistening, emerald green water, and seagulls walked along the sands at the president's feet. But oil is nearby even if it can't be seen, according to Allen.

Onlookers chanted "Save our beach, save our beach."

Addressing the troops at Pensacola, Obama spoke of other daunting challenges facing the nation, telling them that "obviously, the news has been dominately lately by the oil spill but our nation is at war."

And he said the nation has the "strength and resilience" to face down all the different challenges it faces, a message sure to be echoed in his address to the nation.

Gibbs said the reason for wresting the claims-handling process from the British petroleum giant would be to make economically distressed individuals and businesses "whole."

Voicing increasing confidence in his ability to confront the nation's worst environmental crisis, Obama was set to outline a comprehensive response and recovery program, while assuring not only the people from the afflicted region, but all across America, that his administration will guide the country to a recovery.

On the matter of the disputed damage payments, Gibbs said, "We have to get an independent claims process. I think everyone agrees that we have to get BP out of the claims processes and, as I said, make sure that fishermen, hotel owners have a fast, efficient and transparent claims process so that they're getting their livelihoods replaced."

"This disaster has taken their ability to make a living away from them," he said. "We need to do this quickly, and we have to make sure that whatever money goes into that — that in no way caps what BP is responsible for. Whatever money they owe to anybody in the Gulf, they're going to have to pay regardless of the amount."

Obama's address to the nation sets the stage for his showdown White House meeting Wednesday with top BP executives. BP leased the rig that exploded April 20 and led to the leak of millions of gallons of coast-devastating crude. It's part of an effort by Obama, who's been accused of appearing somewhat detached as the oil spill disaster has unfolded, to convince a frightened Gulf Coast and a skeptical nation that he is in command.

The trip gave him ammunition for the speech and for his meeting with BP executives where he intends to finalize the details of a victims compensation fund. He visited vacant beaches in Mississippi where the threat of oil had scared off tourists, heard the stories of local employers losing business, watched hazmat-suited workers scrub down boom in a staging facility in Theodore, Ala., and took a ferry ride through Mobile Bay and then to Orange Beach, Ala., where oil has lapped on the shore.

"I am confident that we're going to be able to leave the Gulf Coast in better shape than it was before," Obama said Monday.

That pledge was reminiscent of George W. Bush's promise to rebuild the region "even better and stronger" than before Hurricane Katrina in 2005. Bush could not make good on that promise, and Obama did not spell out how he would fulfill his. Tuesday's speech will give him the chance.

Presidents reserve the Oval Office for rare televised addresses. When they take their place behind the desk, it's a time for solemnity and straight talk — often a moment of history. There is a sense of gravity. One man by himself before one television camera speaking to the nation.

Oval Office addresses typically aren't lengthy discourses like a State of the Union, but if a president has to go for broke, this is where he does it. Bush addressed the nation from the Oval on the evening of Sept. 11, 2001. Ronald Reagan spoke there after the space shuttle Challenger explosion. John F. Kennedy grimly explained the Cuban missile crisis. Richard Nixon announced his resignation.

Obama hasn't used it yet. Not even during the worst economic crisis since the Great Depression. Not to explain painfully high unemployment rates. Or bank and auto company bailouts. Not to speak of terrorism threats. Even when his health insurance plan was in peril, he did not speak from the Oval Office to rally support or explain to Americans why he considered it vital.