DES MOINES, Iowa —  Democratic Sen. Evan Bayh (search), a possible presidential candidate in 2008, said Thursday that his party lacks credibility on national security and needs to convince Americans that Democrats are willing to use force when necessary.

Until the party can persuade voters, it will be unable to move the debate to issues that work for Democrats, Bayh said in an interview with The Associated Press.

"Unless the American people know that we will be good stewards of the nation's security, they're unlikely to trust us with anything else," said the two-term Indiana senator. "That's a very important threshold we have to get over."

Bayh said there are legitimate grounds to criticize President Bush's approach to fighting terrorism, but until Democrats establish more credibility on the issue, many voters won't listen.

"Many Americans wonder if we're willing to use force to defend the country even under the most compelling of circumstances," Bayh said. "The majority of Democrats would answer that question that, yes, there is a right place and a right time. We don't get to have that discussion because many people don't think we have the backbone."

Bayh has spent three days in Iowa, the first presidential caucus state, attending party fundraisers and meeting privately with activists who play a crucial role in Democratic politics.

Bayh said he would make a decision on a presidential bid after next year's midterm elections, basing it, in part, on whether he has a realistic chance of winning the nomination.

"Is this a sensible thing to do?" he said. "I've never been a big person for fool's errands. I think you have to conclude you have some prospect of being successful."

Bayh said his electoral success in heavily Republican Indiana and moderate views are a model for Democrats to end their recent electoral failures. Summing up those failures are polls that show voters overwhelmingly trusting Republicans on national security, he said.

"We've got a few voices out there who would be a little bit more on the fringe," Bayh said. "Unfortunately, too often they define the entire party."