Las Vegas ? Sen. John Kerry seized upon one of Nevada’s hottest and longest-running controversies Tuesday, vowing to block creation of a nuclear waste dump at Yucca Mountain and accusing President Bush of breaking his 2000 campaign promise to do the same.

The Democratic presidential hopeful has made the environment a focus of his campaign swing this week across two of the most hotly competitive Southwestern states, Arizona and Nevada.

On Tuesday, speaking at a Las Vegas middle school along a proposed truck route to Yucca Mountain, Kerry sought to twin his conservation theme with an assault on Bush’s credibility, suggesting the president had flip-flopped on the issue.

“The fact is the person I’m now running against in this race for president of the United States came here to Nevada, stood up in front of Nevada and made a promise to Nevada that this waste would not come to Yucca Mountain,” Kerry said. “And within weeks and months, that was reversed.”

Yucca Mountain, located roughly 90 miles northwest of this tourist mecca, has been a political issue in Nevada off and on for more than 20 years, as the nation has wrestled with the question of how — and where — to store its deadliest nuclear waste.

In 2002, Bush signed legislation to establish the ridge of volcanic rock and ash as a burial ground for 77,000 tons of highly radioactive waste. Critics say Bush broke a promise he made late in the 2000 campaign when he suggested he was open to other alternatives. Many analysts say that promise was crucial to Bush’s narrow victory in Nevada over Democrat Al Gore, an ardent foe of the project.

A spokesman for the president’s re-election campaign, Steve Schmidt, said Bush had been true to his word.

“The president said his decision would be dictated by sound science, and his policies have been consistent with that promise,” Schmidt said.