Updated

President Bush's credibility in foreign policy has been undermined by questions about how the government used intelligence on Iraq before the war, four Democratic presidential candidates said Sunday.

The four were in Gov. Tom Vilsack (search)'s home town for the governor's annual family picnic, mingling with hundreds of activists in a state where party caucuses next January launch the presidential nominating season.

At the top of their agenda was the suspicion raised in the press and elsewhere about Bush's handling of intelligence data regarding weapons of mass destruction, a primary justification for launching the attack. No weapons have been found despite an intense search since President Saddam Hussein's government was overthrown.

Sen. Joe Lieberman (search), who supported military action in Iraq, questioned whether intelligence agencies "had it right or whether the administration was overstating the case" that Iraq had banned weapons.

Former Vermont Gov. Howard Dean (search) revived a Watergate-era phrase to raise questions about whether Bush withheld information from Congress: "The question now is going to become, `What did the president know, and when did he know it?"

Ohio Rep. Dennis Kucinich (search) said Bush's handling of the issue was fraudulent and demanded a full explanation of reports that some intelligence workers worried that data they were reporting was misused.

"They took this country into a war that we did not have to go into," said Kucinich. "They led this country into a war that was unnecessary."

Florida Sen. Bob Graham (search), former chairman of the Senate Intelligence Committee, one of the first to get into the intelligence fray, accused Bush of "a pattern of deception and deceit of the American people."

One rival for the nomination, Rep. Dick Gephardt of Missouri (search), who supported Bush's war effort, said military action was needed to protect Americans.

"Those questions ought to be answered, because America's credibility is on the line," said Lieberman. He cited pending congressional investigations, saying "those investigations ought to go forward aggressively."

Dean, the former Vermont governor, warned against attacking Bush too harshly.

"I think we ought to take a measured approach to this," Dean said. "Somebody's credibility is on the line, and I would prefer it to be a Republican president's."

In Washington, still another Democratic hopeful, Rep. Dick Gephardt of Missouri, criticized such talk. "You can't bring politics into this. And I felt that from the beginning, and I continue to feel that," Gephardt said on CBS' Face the Nation.

"This is about life and death. It's about keeping this country safe. We cannot have weapons of mass destruction used in this society. And we've all got to work together to the best of our ability to achieve that."