SAN FRANCISCO — Senator Obama is racking up a series of tactical victories in the race for the Democratic presidential nomination, but supporters of Senator Clinton see the ground shifting in their candidate’s favor as Americans grow increasingly focused on the sputtering economy.

The latest spate of good news for Mr. Obama included an endorsement from Senator Leahy of Vermont, a court ruling allowing Las Vegas casino workers to caucus at their workplaces, and an apology from one of Mrs. Clinton’s supporters for alluding to Mr. Obama’s experimentation with drugs as a young man.

However, gloomy news about an economic slowdown and a stock market sell-off has left the Illinois senator defending his ability to take on Mrs. Clinton over economic issues, especially in light of America’s strong economic performance during her husband’s tenure.

“At this point, the contest between myself and Senator Clinton, I think, really does have to do with the past and the future,” Mr. Obama said in response to a reporter’s question after a roundtable here with cash-strapped working women. “The American people are not looking simply for a repetition of what happened in the ’90s. What they’re looking for is who’s going to lead us over the next eight years into an era of great prosperity but will also meet new challenges as they come up.”

Mrs. Clinton’s allies seem eager to push the economic issues to the fore. During a forum in California on Wednesday, her husband presented the problems in almost apocalyptic terms. “The economy is imploding in on itself because we are not producing enough new jobs,” President Clinton said, going on to remind voters of the more than 22 million jobs created while he was in the White House.

“Jump-starting this economy will require a hands-on leader who will manage our economy and hold government accountable for delivering results,” a spokesman for Mrs. Clinton, Blake Zeff, said. He called Mrs. Clinton’s plans to invigorate the economy through a stimulus package, mortgage assistance, tax rebates, and promotion of alternative-energy technology “forward-looking and aggressive.”

The shift to more talk about the economy is a result, in part, of the improving news out of Iraq, and also would seem to portend less discussion of Iraq, at least in the Democratic primary. That, in turn, seems to guarantee fewer public references to Mrs. Clinton’s vote to authorize the war and Mr. Obama’s early stand against it.

The jockeying over the economy came as the Democratic contest in Nevada, which will vote on Saturday, turned sharply negative.

A labor union backing Mr. Obama unleashed $35,000 worth of radio advertisements in Nevada asserting that Mrs. Clinton “doesn’t respect our people.” The ad from Unite Here, airing in Spanish on Spanish-language radio stations, conveys the clear message that Mrs. Clinton has snubbed the Latino community.

“Hillary Clinton’s supporters went to court to prevent working people from voting this Saturday,” the ad says. “This is unforgivable. Hillary Clinton is shameless.”

The ad refers to a lawsuit in which a teacher’s union and several individuals challenged new Nevada Democratic Party rules allowing casino workers to caucus at their worksites. Yesterday, a judge refused to block the casino caucuses. Some of those who brought the suit support Mrs. Clinton, but her campaign has denied any involvement in the case.

Latino backers of Mrs. Clinton were furious about the union ad. “Every single sentence is an attack on Hillary Clinton,” an adviser to the former first lady, Maria Echaveste, told reporters in a conference call. “It’s pathetic and sad that they need to sink so low,” a longtime hispanic labor activist in Mrs. Clinton’s camp, Dolores Huerta, said.

Ms. Echaveste acknowledged that the ad did not come from Mr. Obama, but she said he should denounce it. A spokesman for Mr. Obama had no immediate response. Also in Nevada, Mrs. Clinton unveiled the first negative ad of her presidential bid, a radio spot painting Mr. Obama and another Democratic hopeful, John Edwards, as less-than-staunch foes of plans to store nuclear waste at Yucca Mountain. The ad accuses Mr. Obama of being “hip deep in financial ties to one of America’s biggest Yucca Mountain promoters … nuclear giant Exelon.” The Illinois senator’s presidential bid has taken nearly $200,000 in contributions from people affiliated with the Chicago-based firm, according to federal records analyzed by the Center for Responsive Politics.

“It’s completely ludicrous and disingenuous to suggest that Barack Obama has somehow been soft on Yucca Mountain,” an anti-Yucca activist, Robert Fulkerson, complained in a conference call arranged by Mr. Obama’s campaign. “He’s taken a hard line against Exelon in Illinois.”

While Mrs. Clinton and Mr. Obama have traded negative mailings and fact sheets, they have shied away from negative television ads, which can provoke a backlash. As negative ads go, Mrs. Clinton’s new radio spot is fairly mild. It tangentially suggests something financially unseemly, but the criticism of Mr. Obama comes entirely from a column in the Las Vegas Review-Journal.

The endorsement from Mr. Leahy yesterday has Mr. Obama closing in on Mrs. Clinton in the battle for support among their Senate colleagues. Mrs. Clinton has 10 backers there, while Mr. Obama has seven. However, most of his have come in the past few weeks after he proved his appeal in Iowa.

“We need a president who can reintroduce America to the world and actually reintroduce America to ourselves,” Mr. Leahy said. “Barack Obama represents the America we once were and want to be again.”

Mr. Leahy said he admired both Mrs. Clinton and Mr. Edwards but felt “very, very strongly” that Mr. Obama was the best choice for the presidency.

Mr. Obama also picked up an apology yesterday from the founder of Black Entertainment Television, Robert Johnson, over comments he made which seemed to allude to Mr. Obama’s admission of drug use in his younger days. “In my zeal to support Senator Clinton, I made some very inappropriate remarks for which I am truly sorry,” Mr. Johnson wrote in a letter to Mr. Obama. “I hope that you will accept this apology. Good luck on the campaign trail.”

Mr. Johnson discussed the apology on CNN yesterday but did not explain why he issued a statement on Sunday, through the Clinton campaign, asserting that his disputed comments referred only to Mr. Obama’s work as a community organizer and that any other interpretation was “irresponsible and incorrect.”