With oil selling for more than $130 a barrel in the commodity markets and no end in sight to high gasoline prices, Mr. Bush, a former oilman from Texas who came into office vowing to address an impending energy shortage, does not want to end his presidency in the midst of an energy crisis.

No one knows for certain how much oil is in the moratorium area. The federal Energy Information Administration estimates that roughly 75 billion barrels of oil in the United States may be found in all areas of the country that are now off limits for development, and that 21 percent of this oil  or about 16 billion barrels  is covered by the offshore moratorium.

Mr. Bush’s new stance on offshore drilling will inject him squarely into the presidential campaign, by putting the full weight of the White House behind Mr. McCain at a time when the candidate is trying to demonstrate presidential stature. But it will also expose Mr. McCain to accusations from Democrats that a McCain presidency would be akin to a Bush third term.

At the same time, the move will put the onus on Democrats, many of whom have long been staunchly opposed to offshore drilling. And it is likely to exacerbate the 30-year-old standoff in Washington over whether domestic drilling or conservation is the way to end American dependence on foreign oil.

That debate has grown especially acute in recent weeks, with the White House in “I told you so” mode. In a speech to the United States Chamber of Commerce last week, Vice President Dick Cheney said, “We should hear no more complaining” from opponents of domestic drilling, whom he called “part of the problem.”

Senator Reid responded by calling the vice president “Oil Man Cheney,” saying: “So all that Cheney can talk about, the Oil Man Cheney can talk about, is drilling, drilling, drilling. But there is not enough oil in America to make that the salvation to our problems.”

After hearing of Mr. Bush’s proposal on Tuesday night, Mr. Reid affirmed his opposition, saying, “The Energy Information Administration says that even if we open the coasts to oil drilling that won’t have a significant impact on prices.”