On April 20, as lethal amounts of oil and gas crept into the drill pipe of BP's Macondo well in the Gulf of Mexico, the attention of the energy and environmental world was elsewhere.

The Jefferies investment firm issued a report that recommended buying shares of Halliburton, whose cement job was buckling under the sea floor. Environmental groups were getting ready to celebrate the 40th anniversary of Earth Day. The Obama administration and its Senate allies were planning to unveil a climate proposal alongside corporate leaders, including a top executive from BP.

Then came the Big Spill.

For four months, the geology of the public agenda shifted. The climate proposal was never rolled out. Earth Day's anniversary was the day oil from the Macondo well started gushing into the Gulf of Mexico. President Obama's March push to open areas to offshore drilling gave way to a six-month moratorium on new deep-water exploration wells.

Now that the well is officially dead, what is its legacy? Will the massive spill -- the biggest offshore spill in U.S. history -- make a drop of difference in the long run? Will it change long-term trends in offshore oil drilling, U.S. energy policy or the way consumers use energy?

"For the companies, the regulators and the gulf residents, the spill has had a profound impact on their operations and ways of life. For the broader national population and the political debate, it has had a surprisingly small impact," said Jason Grumet, executive director of the National Commission on Energy Policy, a part of the nonprofit Bipartisan Policy Center. "It is the modesty of the impact on the national energy debate that is in many ways most surprising."

For a time, it seemed that the spill might galvanize lawmakers and consumers. But consumers respond to gasoline prices, which have been unusually stable for the past few months. In the Senate, ironically, a climate compromise unraveled, in part because expanded offshore drilling could no longer be used to woo undecided members. The Senate probably won't even pass a narrow oil spill measure, which has been bogged down in disputes over such issues as liability limits for offshore drilling.

Moreover, Obama is expected to lift his drilling moratorium soon, yielding both to pressure from Gulf Coast lawmakers worried about jobs and to the reality that offshore drilling is needed to meet U.S. oil demands. Chernobyl helped end nuclear plant construction in Europe, but deep-water offshore drilling will continue because that's where the oil is. Now, as the National Oceanic and Atmospheric Administration declares that much of the spill has been devoured by microbes and dissipated, the political impact of the spill could dissipate further.

"As soon as the well was contained, we bounced back to American exceptionalism," said Grumet. It was as though, he said, the nation cried, "We did it!"

"Is it the Twitter consciousness where every 15 seconds we have a new thought?" Grumet wonders. "Is it that the recession has made so many people worried about themselves? Or is it that the impacts were so localized that once the newspapers stopped talking about it, it went away?"

But many experts said that, within the industry, the spill has had lasting consequences, apart from any direct environmental damage.

"I think the spill is a game changer," said William Reilly, co-chairman of the president's oil spill commission tasked with examining policy for offshore drilling in light of what went wrong with the Macondo well. "The industry has never been more alert to safety."