In American politics, the rallying cry “No nukes!” seems to be losing its punch.

The crop of candidates seeking the White House in 2008 shows an affinity for atomic energy that would have been unthinkable a decade ago.

“You absolutely would not have gotten the same reaction not that long ago,” a key anti-nuclear activist in California, Rochelle Becker, said.

Each of the top contenders for the Republican nomination and all but one of the major Democratic hopefuls support nuclear power to some extent. Most cite the prospect that atomic energy could help reduce climate change by supplanting power produced by fossil fuel sources such as coal and natural gas.

“The global warming issue is what is causing at least the Democratic candidates to say we need to leave nukes on the table,” Ms. Becker, the executive director of the Alliance for Nuclear Responsibility, said.

The two leading Democratic presidential candidates, Senators Clinton and Obama, have joined one of the top Republicans in the race, Senator McCain of Arizona, to sponsor the Climate Stewardship and Innovation Act of 2007. The measure includes more than $3.6 billion in funding and loan guarantees for the planning and construction of nuclear plants using new reactor designs.

The only major candidate opposed to increased reliance on nuclear power is a former senator from North Carolina, John Edwards. The Las Vegas Review Journal reported that during a visit to that city in February, Mr. Edwards declared that atomic energy had no future in America. A spokeswoman for the candidate, Kate Bedingfield, said the report slightly overstated his position, but she added, “He does not advocate building additional nuclear power plants in the U.S.”

One potential entrant in the Democratic field, Vice President Gore, also remains cool to nuclear power. Despite his advocacy for urgent action to combat climate change, he has repeatedly dismissed the prospect of increased reliance on atomic power.

The Democrats’ take on nuclear energy this time is nearly a mirror image of their stance in 1992. Then, a former senator from Massachusetts, Paul Tsongas, was the sole advocate for greater use of atomic power. The rest of the field stood opposed and heaped scorn on Tsongas as a proponent of a dangerous and foolish policy.

“There is no such thing as a pro-nuclear environmentalist,” one of Tsongas’s rivals, Senator Harkin of Iowa, declared in campaign ads. Other Democratic hopefuls such as a former California governor, Jerry Brown, and a senator from Nebraska, Robert Kerrey, made similar arguments.

However, the leader of the anti-nuclear assault on Tsongas was arguably the then-governor of Arkansas, Bill Clinton, who was jousting with the Massachusetts native for the front-runner slot in the presidential nominating contest.

“We do not need to do what Senator Tsongas needs to do and build hundreds of more nuclear plants to become energy independent,” Mr. Clinton said at a debate in Denver on February 29, 1992. “One of the reasons he’s ahead in the polls it that people do not know what he stands for.”

Tsongas took umbrage at the claim that he wanted to build hundreds of new nuclear facilities. “That is a lie. That is a lie. That is a lie,” the former senator declared.

The fight over the nuclear issue provoked one of the primary campaign’s most memorable exchanges, as Mr. Clinton went on to needle his rival by saying, “No one can argue with you, Paul, you’re always perfect.”

“I’m not perfect, but I’m always honest,” Tsongas replied caustically.

Mr. Clinton’s use of the nuclear cudgel against Tsongas is in sharp contrast to Mrs. Clinton’s open embrace of nuclear power in the current campaign.

“I think nuclear power has to be a part of our energy solution,” the New York senator said during a town hall meeting in Aiken, S.C., in February. “We’ve got to be very careful about the waste and about how we run our nuclear plants, but I don’t have any preconceived opposition. I just want to be sure that we do it right, as carefully as we can because obviously it’s a tremendous source of energy. We get about 20% of our energy from nuclear power in our country. … Other countries like France get, you know, much, much more. So, we do have to look at it because it doesn’t put greenhouse gas emissions into the air.”

Mr. Obama’s camp gave a somewhat more reserved answer when asked about the Illinois senator’s views on atomic energy. “Barack Obama feels we must address three key issues before ramping up nuclear power, including the public’s right to know, security, and waste storage,” a campaign spokeswoman, Jennifer Psaki, said. “Nuclear power represents the majority of non-carbon generated electricity therefore making it unlikely that it will be taken off the table.”

A spokeswoman for another Democratic hopeful, Senator Dodd of Connecticut, said he “has been supportive of safe nuclear power.” The campaign of Senator Biden of Delaware did not respond to requests for comment on the issue, but in 2005 he voted for an amendment containing a next-generation nuclear design program similar to the one backed by Mrs. Clinton and Messrs. McCain and Obama.

Tsongas, the early nuclear advocate, died at 55 in 1997, from a recurrence of lymphoma. His adviser on energy issues, Mitchell Tyson, said Tsongas backed nuclear power primarily as an alternative to the pollution caused by burning coal. However, press clippings show that, even back in 1992, the Lowell, Mass., native also cited global warming as a reason to consider atomic power.

“Paul Tsongas was 100% right,” Mr. Tyson said in an interview this week. “Paul had a multigenerational approach to energy. He did not see it as what’s easy today. … It’s nice to see the mainstream Democratic candidates coming around to it.”

When read the text of Mrs. Clinton’s recent remarks, Mr. Tyson said he was stunned by the contrast with her husband’s anti-nuclear attacks a decade and a half ago. “Amazing. Just astounding,” he said.

One critical part of the nuclear calculus for Democrats these days is the negative sentiment of Nevada residents to the federal government’s plan to store high-level nuclear waste at a site there known as Yucca Mountain. The clout of Nevada voters is magnified in this cycle by plans to stage the state’s Democratic presidential caucuses on January 19, 2008, prior to New Hampshire’s primary.

The four senators in the Democratic race also have another good reason not to get crosswise with Nevadans: the Senate majority leader, Harry Reid, hails from that state.

“In the Democratic Party in Nevada, it’s just sort of an article of faith that you don’t even question that Yucca is bad,” a political science professor at the University of Nevada at Las Vegas, Eric Herzik, said.

As a result, and perhaps for substantive reasons as well, all of the major Democratic candidates are now opposed to the plans for Yucca. The even appear to be in something of a competition to outdo one another on the issue. Mr. Biden’s campaign sent out a press release Monday calling attention to a Washington Post report that found him to be a “steady foe” of Yucca but labeled Messrs. Dodd and Edwards as “flip-floppers.” The article said Mrs. Clinton has also steadily opposed Yucca for a number of years.

Mr. Dodd backed the waste site in 2002 but recently said he opposes it because of security concerns. Mr. Edwards voted against the Yucca plan in 2000, in favor of it in 2002, and is now back to opposing it. An aide said the former senator said he was troubled by recent allegations of forged engineering reports and by increased prospects that terrorists could intercept waste shipments.

Anti-nuclear activists like Ms. Becker see hypocrisy in those who claim to be worried about the waste but still tout nuclear power as part of the solution to global warming.

“If you’re really concerned about the waste, how can you favor nuclear power if we have no way to deal with the waste?” she asked. “What they’re doing is really a very political decision and not a very pro-nuclear decision.”