Washington ? Shaking up years of energy policy and his own environmental backers, President Barack Obama threw open a huge swath of East Coast waters and other protected areas in Alaska and the Gulf of Mexico to drilling Wednesday, widening the politically explosive hunt for more homegrown oil and gas.

Obama’s move allows drilling from Delaware to central Florida, plus the northern waters of Alaska, and exploration could begin 50 miles off the coast of Virginia by 2012. He also wants Congress to lift a drilling ban in the oil-rich eastern Gulf of Mexico, 125 miles from Florida beaches.

Still off limits: the entire Pacific seaboard. And in a nod to conservation, Obama canceled oil exploration in Alaska’s Bristol Bay, deeming the area a national treasure.

For this oil-dependent nation, the decision could start to reshape far-reaching economic and national security policies, affecting where the U.S. gets the fuel for its cars, heating and energy-gulping industry.

For a president on a roll following a big health care win, Wednesday’s drilling declaration was both aggressive and pragmatic.

Even with a push for cleaner energy sources and efficient cars — and with promises of protection for ecosystems and coastal tourism — the nation still needs more oil, Obama said.

“The answer is not drilling everywhere all the time,” Obama said in an event at Andrews Air Force Base. “But the answer is not, also, for us to ignore the fact that we are going to need vital energy sources to maintain our economic growth and our security.”

Inside politically conscious Washington, Obama’s announcement was viewed, too, as a play to win Republican support for a comprehensive climate change bill. Obama needs GOP help to move legislation through the Senate that would limit carbon emissions, a key priority, and his decision on drilling drew at least a bit of Republican applause.

Republican George W. Bush pushed for years to expand offshore drilling. He and Congress lifted bans on some drilling in 2008, when gasoline prices hit record levels. But Obama’s plan is narrower than Bush’s, which also would have opened up oil and gas leasing areas off California and in the North Atlantic.

Obama got a predictable pummeling Wednesday from environmentalists, who sarcastically compared him to Sarah Palin, the former vice presidential candidate whose oil-promoting speech at the Republican National Convention in 2008 famously drew chants of “Drill, Baby, Drill!”

Any big changes to environmental policy — particularly oil drilling — tend to touch off the bitter debate that Obama says he wants the country to end.

His support for exploratory drilling in the Chukchi and Beaufort seas north of Alaska, for example, drew outrage from the Center for Biological Diversity as a threat to polar bears. “Short of sending Sarah Palin back to Alaska to personally club polar bears to death, the Obama administration could not have come up with a more efficient extinction plan for the polar bear,” said Brendan Cummings, the center’s senior counsel.

More broadly, the conservation group Oceana declared Obama was “unleashing a wholesale assault on the oceans.”