Senator Barack Obama might be passing his days on the volcanic islands of Hawaii, but his staff back home is focusing on another mountain: Yucca. On Saturday, the campaign released a new television spot about the potential storage site for nuclear waste, which, according to the release, will hit airwaves in Las Vegas and Reno on Monday.

The ad cites Senator John McCain’s support for storing nuclear waste in Nevada’s Yucca Mountain, then shows an interview clip of Mr. McCain saying he would not be comfortable with nuclear waste traveling through Arizona, and specifically Phoenix, on the way to the storage site.

“John McCain. For nuclear waste in Nevada, just not in his back yard,” the announcer says. “Barack Obama. Opposes Opening Yucca. He’ll protect our families.”

The timing of the spot is intended to coincide with Mr. McCain’s campaign visit to Nevada this weekend. He speaks at the Disabled American Veterans convention in Las Vegas Saturday.

Mr. McCain’s campaign responded by pointing out that Mr. Obama voted twice for a bill in 2005 that would have provided money to fund the disposal site at Yucca. “Apparently Barack Obama is also taking a vacation from the facts, ignoring his own votes in support of the Yucca Mountain project,” said Tucker Bounds, a McCain spokesman, in a press release. “Either Barack Obama is too inexperienced to understand that his votes on the floor of the United States Senate are recorded for Americans to review, or he’s simply showing incredible hypocrisy.”

However, the bill is, in fact, a sweeping appropriations bill for energy and water development. Three senators voted against the measure (five missed the vote) – Mr. McCain was among those voting nay.

Speaking out on Yucca has made Mr. Obama vulnerable to criticism in the past. After Mr. Obama joined fellow Democrats in expressing opposition to using the storage facility before the Nevada caucuses in January, Senator Hillary Rodham Clinton’s campaign questioned his credibility on the issue, pointing to his financial support from employees of Exelon Corporation, the country’s largest nuclear operator, which supports using Yucca Mountain as a waste site.

“What part of ‘I’m not for Yucca’ do you not understand?” said an exasperated Mr. Obama at the time.

The Times’s Mike McIntire reported on Mr. Obama’s experience with Exelon in February.