"The selection of Yucca Mountain has failed, the time for debate on this site is over and it is time to start exploring new alternatives for safe, long-term solutions based on sound science," Obama said in the letter.

The candidate clamber underscores how they perceive Yucca Mountain as a major litmus test as Nevada Democrats prepare to hold their early presidential nominating caucuses next January, said Eric Herzik, a political scientist at the University of Nevada, Reno.

"Everybody is opposed to Yucca Mountain when they talk in Nevada, and talk to Nevadans," Herzik said. "In the Democratic primary, you have to be opposed."

But Herzik said the efforts by candidates to "out-Yucca" one another could lessen the impact of today's hearing, where Democrats want to showcase tough questions to the Department of Energy and other federal agencies.

"This is symbolic politics, and it is addressed to Democratic voters," Herzik said. "It is being perceived as a political show."

Clinton has worked to position herself to Nevadans as the candidate most likely to kill the Yucca project if elected president. All Democrats seeking votes in the state's early nominating caucus next January say they oppose the repository.

But Edwards moved to slow the Clinton charge, saying Nevadans should take a closer look at her. Although Clinton opposes Yucca Mountain, Edwards referenced remarks she made at an August debate where she said she was "agnostic" on the larger issue of nuclear power.

"Americans need a committed leader who will challenge the system and bring change, not someone who is still making up her mind," Edwards said in a statement released by his campaign on Tuesday.

"Senator Clinton again is trying to have it both ways by asking for a hearing to delay the Yucca Mountain project while declaring herself agnostic on nuclear energy," Edwards said.

More nuclear power plants mean more nuclear waste, and growing needs to dispose of it, said Edwards, who says he opposes building any new reactors "until we find another way to permanently and safely store nuclear waste."

In the meantime, Edwards said, "we can meet our energy needs through renewable energies, improved energy efficiency and the use of coal with permanent storage of its carbon emissions."

Responding to the charge, Clinton brought up Edwards' own baggage on Yucca Mountain. As a North Carolina senator in 2002, he voted in favor of President Bush's decision to designate the Nevada nuclear waste site.

"Rather than attack Senator Clinton, a consistent opponent of Yucca Mountain, perhaps he should spend his time explaining to Nevadans why he voted for it," said Hilarie Grey, a Clinton campaign spokeswoman.

"Rather than merely talking about Yucca Mountain, Senator Clinton is taking action," Grey said, adding that Edwards is going on the offensive because his campaign is struggling.

Edwards has explained his shift on Yucca since 2002. He said it became clear to him that the science supporting nuclear waste burial is Nevada "is unreliable," and that he also grew concerned about transporting waste across the country and the threat from terrorists.

Obama was preparing a statement to be submitted to Sen. Harry Reid, D-Nev., and to Sen. Barbara Boxer, D-Calif., chairwoman of the Senate Environment and Public Works Committee that is sponsoring the hearing. A copy was made available on Tuesday.

In it, Obama said the likelihood that Yucca Mountain will continue to be challenged on scientific and legal grounds, and may not open for decades if ever, means the government should look for alternatives.

"The time has come for the federal government to refocus its resources on finding more viable alternatives for the storage of spent nuclear fuel," he said, including looking for another state willing to host a repository.

"The federal government should also redirect resources toward improving the safety and security of spent fuel at plant sites around the country until a safe, long-term solution can be implemented," he said.

Contact Stephens Washington Bureau Chief Steve Tetreault at stetreault@stephensmedia.com or (202) 783-1760.