Democratic presidential hopeful Dick Gephardt (search) took his sharp criticism of Howard Dean (search) from the campaign trail in Iowa to the television airwaves Thursday, broadcasting a commercial that attacks him on Medicare and Social Security.

"How much do you really know about Howard Dean?" an announcer asks in the ad, which includes various clips of Dean talking. "Did you know Howard Dean calll Security."

The 30-second spot is meant to energize Gephardt's core supporters and tap into anti-Dean sentiment from voters who may be taking a second look at the former Vermont governor's candidacy. Dean and Gephardt are in a close fight in Iowa, which holds its caucuses Monday, along with John Kerry (search) and John Edwards.

Dean dismissed the ad.

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"These ads that go on in the last minute are completely silly ... We're not going to even get into that. We're going to run a positive campaign. You won't see us running any ads that are anything like that because it's silly," Dean told reporters while campaigning in Carroll, Iowa.

However, Dean is running an ad in Iowa that criticizes Gephardt, Kerry and Edwards for supporting the congressional resolution authorizing the war against Iraq. Pressed on that, Dean said: "All we're doing is laying out the facts. There are some negative ads on. Ours are not one of them."

The ad wars heated up this week with Dean's Iraq commercial. On Thursday, he also was to start running a closing ad asking Iowans for their support. And, Kerry planned to roll out his last Iowa ad Friday, featuring the endorsement of Iowa's first lady. Edwards' final ad includes a clip of his endorsement from the Des Moines Register.

Gephardt is taking a risk by airing a spot that assails his rival because Iowa, the Missouri congressman's must-win state, traditionally rewards candidates who don't go negative on the stump or on TV. The ad could backfire by causing potential Gephardt voters to support Kerry and Edwards, whose ads are positive and who are trying to stay above the fray. Dean also could use the attack to mobilize his own base, which he has done in the past.

Gephardt has questioned Dean's position on Medicare and Social Security -- and comments he has made about the federal programs -- throughout the campaign. He did it again Wednesday in a speech that assailed Dean on several fronts.

Iowa Democratic Sen. Tom Harkin, who has endorsed Dean, said the Gephardt ad distorts Dean's positions.

"They've just gone overboard on Howard Dean. They've taken things out of context. I know Howard Dean. I mean, to leave the impression he's opposed to Medicare? That's just bizarre," Harkin told reporters while traveling with Dean on a bus tour of Iowa.

Dean's new ad is meant to ensure his supporters participate in Monday's caucuses.

"Our campaign doesn't just talk about change," Dean says. "We're empowering the American people, so that, together, we can demand health care for all and a government that works for people again. That's what's at stake on Monday. So please, don't stay home."

Kerry's new ad features Christie Vilsack, the wife of Democratic Gov. Tom Vilsack, who has said he will not endorse a candidate before the caucuses. In the ad, the first lady tells viewers why she supports the Massachusetts senator and names newspapers that have endorsed him.

"I'm standing with John Kerry because he'll fight for us. He has a lifetime record of taking on special interests," she says.

Meanwhile, Joe Lieberman, who is not competing in Iowa, has started running radio ads in Delaware and South Carolina, featuring political leaders from each state. Sen. Tom Carper in Delaware and three state senators in South Carolina urge voters to choose the Connecticut senator for president. One of the ads, which are the first to mention Lieberman's religion, is running on gospel stations.