Barack Obama pledged Wednesday to place new emphasis on ridding the world of nuclear weapons if he is elected president, warning that nuclear terrorism is the "gravest danger" facing the country.

The presumptive Democratic presidential nominee was picking up where he left off during a foreign policy address of a day earlier, when he stressed the need to end the war in Iraq so that the country can devote its resources to fighting terrorism in Afghanistan and preventing nuclear weapons from falling into the hands of rogue regimes.

At a national security summit in West Lafayette, Ind., Wednesday, Obama said the United States is always responding to threats that have already come to fruition, in effect, "constantly fighting the last war."

Obama said the nation needs to take a more preventive role in addressing threats, and that two goals of his administration would be to secure all loose nuclear material during his first term, as well as rid the world of nuclear weapons.

The United States will "retain a strong deterrent" as long as nuclear weapons exist, he said, but "we will make the goal of eliminating all nuclear weapons a central element in our nuclear policy."

"It's time to send a clear message to the world: America seeks a world with no nuclear weapons," Obama said.

The Illinois senator said the U.S. would negotiate with Russia to achieve "deep reductions" in both countries' arsenals and work to "reduce global stockpiles dramatically."

He did not comment on the news that the U.S. was sending a top diplomat to Geneva to attend talks with Iran's nuclear negotiator. However, he issued a statement saying he welcomed the news "that the Bush administration has shifted course and will send an envoy for direct talks ... with the Iranians in Geneva this weekend."

Foreign policy adviser Susan Rice also said on an earlier conference call Wednesday that the Geneva trip is a "positive step."

The John McCain campaign issued a statement saying the trip represented a "much more realistic approach than engaging in the unilateral cowboy summitry advocated by Senator Obama."

At the Indiana forum, Obama repeated that he supports direct diplomacy with Iran.

"By keeping our commitment under the Nuclear Non-Proliferation Treaty, we'll be in a better position to rally international support to bring pressure to bear on nations like North Korea and Iran that violate it."

Among those joining Obama for the panel discussion were two potential running mates, Sen. Evan Bayh, D-Ind., and former Sen. Sam Nunn, D-Ga.

As the former governor of a Republican state, Bayh could help Obama. Nunn, a defense expert from the South, would burnish the ticket's experience.

When asked if he were interested in the job or had provided material to vetters, Bayh repeatedly referred reporters to the Obama campaign. Nunn said he thought an Obama-Nunn ticket was unlikely.

"If anyone offered me any high office in U.S. government, I'd be greatly honored and I'd talk to him. Certainly I would talk to Sen. Obama if he wanted to talk about it, but I think the chance of an offer are pretty slim," Nunn said.

In his remarks, Obama also paid tribute to Sen. Richard Lugar, R-Ind., who has focused on nuclear non-proliferation issues for much of his career, working closely with Nunn.

Indiana is a Republican-leaning state but Obama hopes to put it in play in the general election, capitalizing in part on his neighbor status from Illinois.

The senator also called for investing in methods to prevent, detect and contain biological attacks. He highlighted a proposal to spend $5 billion over three years to develop an international intelligence and law enforcement infrastructure to stymie terrorist networks.

He also addressed cyber-threats, pledging to appoint a national cyber adviser who will coordinate government efforts and report directly to the president.

The event continued the buildup for Obama's upcoming visit to Iraq and Afghanistan. He also plans to travel to Jordan, Israel, Germany, France and Britain during his overseas trip.

The Associated Press contributed to this report.