Democratic presidential hopefuls who struggled to find a unified foreign policy voice at the height of the U.S.-led war against Iraq now are trying to exploit what they see as a George W. Bush credibility gap.

An e-mail that the Democratic National Committee (search) sent Thursday to more than a million party faithful appealed for donations to pay for an ad that sharply questions President Bush's veracity on Iraq's weapons as several of the candidates stepped up their criticism of the commander in chief.

Sen. John Kerry (search) of Massachusetts, who backed the Iraqi war, argued that Bush has not been honest with the American people about the continued threat facing U.S. forces and the imperative of appealing for international help -- and he made his point by raising the specter of the years the U.S. military struggled in Southeast Asia.

"I learned a long time ago in Vietnam what happens when pride gets in the way of making honest decisions," Kerry, a decorated Vietnam War veteran, told a Capitol Hill news conference. "We carried that war on for too many years because of pride. And I refuse to see us now put American soldiers in risk because we are unprepared to say, 'Where the world is prepared to be part of this, winning the peace in Iraq is not just an American interest, it's a global interest."'

Kerry's campaign strategy in the next few weeks will be to focus on what he contends is a pattern of deception by the administration on several issues beyond Iraq and the war, a campaign official said.

The White House acknowledged earlier this week that Bush was incorrect to say in his State of the Union address that Iraq had sought significant quantities of uranium in Africa, a substance that could be used to restart a nuclear weapons program.

Campaigning in New Hampshire, Democrat Howard Dean (search) demanded the resignation of any Bush administration official or federal government employee who failed to tell the president that claims about Iraq buying uranium from Africa were false.

"We do not know who these senior officials are, but the president should have been given that information," Dean told a group of reporters outside a hospital in Derry, N.H. "The individuals who misled the president know who they were and they should resign immediately."

The former Vermont governor, who opposed the war, added: "The only other possibility, which is unthinkable, is that the president of the United States knew himself that this was a false fact and he put it in the State of the Union anyhow. I hope for the sake of this country that did not happen."

Asked whether he thought Vice President Dick Cheney should resign if he knew, Dean said, "Anybody who misled Bush should leave office, whoever that may be."

Pressed on whether Bush should resign, Dean said, "I think before we cross to that we better find out what the facts are."

But in an echo of Watergate and a Republican president who did resign, Richard M. Nixon, Dean said, "The time for stonewalling is over."

Another candidate, Sen. Bob Graham of Florida, said he holds Bush responsible for using the erroneous information. "I believe in the old admonition if you're the captain of the ship and the ship goes aground, you're responsible," he said on CNN's "Inside Politics."

The Republican National Committee dismissed the criticism, with senior adviser Ed Gillespie saying Democrats were tripping over themselves to get to the left of the Dean.

"Their politics may appeal to their anti-war base, but their lack of policy won't make our country more secure," he said.

Kerry, Dean and other Democrats running for president have called for an investigation, similar to the argument made in the DNC e-mail and the accompanying video that the Democrats hope to run as an ad.

"In his State of the Union address, George W. Bush told us of an imminent threat ... America took him at his word," the video says, showing Bush delivering the speech as it aired on Fox News. "But now we find out it wasn't true."

The ad continues, "a year earlier, that claim was proven false. The CIA knew it. The State Department knew it. The White House knew it."

Not only does the e-mail solicit money so the party can buy television time to air the ad, it urges Democrats to push for an independent, bipartisan investigation of Bush's claims about Iraq's weapons of mass destruction.

"It's time to tell the truth," the ad said.

Democratic Party spokesman Tony Welch said when and where the ad will be aired depends on the response the party gets.

The RNC said it was unfortunate that the Democrats were using national security and the war to raise money.

"While the president is working to make our nation and our world more secure, the Democrats are making videos," said Christine Iverson, a spokeswoman for the Republican National Committee.

On Capitol Hill, several congressional Democrats sharply assailed Bush's effort to seek international backing before the war and reach out to allies now to ensure peace.

Sen. Robert C. Byrd, D-W.Va., an outspoken critic of the war, said the "tragic failure of the administration's efforts to build international support before launching its impatient rush toward war against Iraq is now bearing its bitter, bitter fruit."

Sen. Joseph Biden, D-Del., who is weighing a bid for the presidency, called for an international mix of post-war troops in Iraq, and said Bush "has been missing in action in explaining to the American people why its important to stay in Iraq."