SANTA CLARA, Calif. — A former senator considering a second bid for the White House, John Edwards of North Carolina, said yesterday that re-establishing America’s moral pre-eminence abroad will be the greatest challenge facing the nation’s next president.

“We have issue after issue after issue that are very important here at home, but the overriding responsibility of the next president is to try to restore America’s leadership in the world,” Mr. Edwards said during an appearance before the Commonwealth Club of Silicon Valley to promote a book that he edited, “Home: The Blueprints of Our Lives.”

“We didn’t used to be the country of Guantanamo and Abu Ghraib,” the former senator said, referring to the prison for terror suspects at an American military base in Cuba and the jail in Iraq where American military personnel tortured and sexually humiliated detainees. “We were different. We were the defender of human rights. We were the country that everyone looked up to and respected, and I want to see us be back in that place,” he said.

Mr. Edwards warned that civilization itself might unravel if America cannot recover its moral position. “I don’t believe it’s too much to say the future of the world is at stake. I do — I believe that,” he said.

Part of Mr. Edwards’s argument seemed to be directed at conservatives who have asserted that America can win respect through military might and that leaders should not be overly concerned about employing the nation’s power. “You don’t lead just by being powerful. It takes more than that,” the former senator said.

Mr. Edwards suggested that the divergent responses abroad to crises in Iraq, Iran, and North Korea were the result of Bush administration policies that squandered America’s reputation as a beacon of liberty. “When there’s no leadership, when crises occur, you see this unstable chaotic frenzied response, which is exactly what we’ve been seeing,” the former senator said. “People resist us. Other countries resist us instead of rallying around us.”

Mr. Edwards seemed particularly concerned about America’s failure to bring an end to the killing in the Darfur region of Sudan. “What have we done about it? Nothing. How can the United States of America claim the platform of moral leadership when we stand by and let a genocide take place?” he asked.

Mr. Edwards’s presidential bid in 2004 foundered in part because of a perception that the one-term senator was a novice when it came to national security matters and foreign affairs. He was coy yesterday about his plans for 2008, but he emphasized that since leaving the senate two years ago, he has been doing his international homework. “I’ve spent a lot of time in the last few years overseas, and engaged in some causes that I care about, and meeting with leaders and people,” he said.

Mr. Edwards said he recently traveled to China and found that American policies undercut his efforts to fight human-rights abuses and promote environmental responsibility in that country. New Chinese coal-plants are causing “extraordinary damage to the ozone, and it’s getting worse and worse and worse,”he said.

While Mr. Edwards made some references to housing and the needs of the poor, he did not repeat the central “two Americas” theme from his 2004 campaign, in which he said the country was divided between haves and have-nots. Yesterday, the former senator decried “politicians who demonize people and create toxicity,” including along economic lines.

“We shouldn’t be against rich people. We shouldn’t be against poor people,” he said.

Mr. Edwards also said a campaign finance regulation law that he supported, known as McCain-Feingold after its main sponsors, has proved to be a failure. “If it’s had any effect, I can’t tell it,” he said. The former senator said he now supports public financing for all political campaigns.

Several people among the hundreds who turned out yesterday to hear Mr. Edwards and have books signed said they were ready to sign on if he runs again for the White House.

“He has a genuine integrity,” a retired librarian from Fremont, Calif., Mary Rodriguez, 70, said. “He practices what he preaches.”

Asked about Senator Clinton’s prospects, Ms. Rodriguez said, “She’s carrying too much baggage. It’s a pity.”

Another convert for Mr. Edwards, Ernest Kaplan, 67, of Bayside, Calif., said he thought Mrs. Clinton could not form the connection needed to win over voters. “Intellectually, I agree with her, but there’s something just on a gut level that I can’t relate to,” he said.

The comments of one attendee suggested that Mr. Edwards may have retreated a little too far from his “two Americas” theme. “His presentation was a little too mild for me,” Ursel Bloxson, 71, of Fremont, Calif., said. She also faulted him for failing to say what he would do about the Darfur genocide.

Separately, another potential contender for the Democratic presidential nomination, Senator Bayh of Indiana, said he plans in the next few days to make his first formal move toward a White House bid.

“Later this week, I will be forming an exploratory committee to take the next practical step,” Mr. Bayh said yesterday on ABC’s “This Week.” “I’ll make a final decision over the holidays with my family and have some final decision to announce early next year,” he said.

Asked how he would compete with better-known potential candidates from both parties, including Mrs. Clinton, Mr. Obama, and Senator McCain of Arizona, Mr. Bayh said he believed he could overcome any disadvantage through retail politicking in states such as Iowa and New Hampshire, which are host to early primaries and caucuses. “As the people get to know me, I think we’ll do very well,” Mr. Bayh said. “Is this a little bit like David and Goliath? A little bit, but, as I recall, David did okay.”