WASHINGTON (CNN) - Opponents of the $31 billion Republican energy bill blocked its passage in the Senate Friday morning, throwing the future of the President's energy policy -- a main plank of his domestic agenda -- into grave doubt.

The bill's backers narrowly failed to win the 60 votes needed to overcome a filibuster led by Sen. Charles Schumer (D-N.Y.), with six rebel Republicans providing the key votes.

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The final vote was 57 to 40, but Senate Majority Leader Bill Frist (R-Tenn.) said Republicans will keep trying.

"I do want to let colleagues know this will not be the last vote," said Frist. "We are going to keep voting until we pass it and get it to the President's desk."

Backers and opponents of the bill both said a provision providing protection from lawsuits for firms that make and distribute MTBE, a gasoline additive that has been found to contaminate groundwater, turned opinion against the bill.

"This bill is a full-scale retreat when it comes to environmental protection for America," said Schumer. "To walk away from basic environmental protection in the name of promoting energy is a bad deal for America's future."

Energy Committee Chairman Sen. Pete Domenici (R-N.M.) said that the bill's opponents have made future blackouts, like the kind the Northeast suffered over the summer, more likely.

"The blackouts in America will remain alive and possible because we will have thrown out the window the reliability standards that are in this bill because some want to make the case on an issue like MTBE or the like," said Domenici. "If you like blackouts, then you vote to kill this bill."

While the Democrats were chiefly opposed to a provision providing protection from lawsuits for firms that make and distribute MTBE, Republicans focused in on the enormous amount of pork stuffed into the bill.

"You can't claim to be a fiscal conservative and support the profligate spending and corporate welfare in this bill," said Sen. John McCain (R-Ariz.). He called the bill a "twelve hundred page monstrosity chock full of special interest giveaways."

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A group of farm state democrats, including Senate Minority Leader Tom Daschle (D-S.D.), reluctantly supported the bill because of a provision that would have doubled domestic production of corn based ethanol, a high priority for corn farmers. But Daschle also had serious reservations about the bill and its impact on the environment, and did not pressure his colleagues to get behind it.

The bill was born in secret energy task force meetings headed by Vice President Dick Cheney and authored by Capitol Hill Republicans with little input from Democrats.  

-- from CNN Congressional Producer Steve Turnham