DES MOINES, Iowa (Reuters) - The campaigns of two Democratic presidential contenders accused front-runner Howard Dean's camp of political dirty tricks on Thursday, including plans to have out-of-state supporters pose as Iowans and participate in the Jan. 19 caucuses.

Richard Gephardt's campaign manager, Steve Murphy, said a Dean field organizer told a Gephardt staff member that some of the expected 3,500 out-of-state Dean supporters coming to Iowa to turn out the caucus vote would try to infiltrate the process.

Dean campaign manager Joe Trippi denied the accusation and told Murphy in a letter that "sleazy tactics like yours are exactly the reason that people have stopped participating in the political process."

John Kerry's Iowa state director, John Norris, also said two out-of-state Dean supporters posing as undecided Iowans had tried to get information about campaign voter calling scripts from a Kerry office in the southeastern town of Creston.

The heated exchanges underlined the high stakes in the caucuses, the first key prize in the race for the Democratic nomination and the right to challenge President Bush in November.

Dean is leading recent polls in the state, with Missouri congressman Gephardt running close behind and Kerry, a Massachusetts senator, in third.

"It has come to our attention that your campaign in Iowa is engaged in an effort to violate caucus rules and send out-of-state supporters to pose as Iowa residents and caucus in cities and towns across the state," Murphy said in a letter to Trippi.

He later told reporters the effort was "a direct challenge to the integrity of the caucuses" and called on Trippi to identify and fire the individuals responsible for it.

Norris made his accusations in a letter to Dean's Iowa state director, Jeanni Murray, and called on her to fire the two men, one of whom he named. He said the man admitted to Kerry staffers that he was a Dean supporter.

"This is not how we campaign in Iowa," Norris said. "How can any Iowa voter trust that your campaign will prevent volunteers from engaging in the same illegitimate activity on January 19?"

MONITORING PLANNED

Murphy said the Gephardt campaign, which faces a must-win battle with Dean in Iowa, planned to step up its monitoring of caucus participants and had asked the state party to be particularly vigilant.

Participants in the caucuses, sponsored by the state Democratic Party, must be registered Democrats who will be old enough to vote in November, but they can register on the spot and identification or proof of residence is not required.

Since it is a party event, Murphy said, there is no legal penalty attached to the fraud and he would not challenge the results if Gephardt lost.

State party officials sent a warning to the campaigns in November after a Dean staff member in Vermont called and asked if a hotel address was sufficient grounds to participate. At the time, Dean officials dismissed the significance of the call and attributed it to a teen-age intern.

Trippi, who worked for Gephardt in 1988 as a deputy campaign manager, said the latest charge was "ridiculous" and that voters were tired of "this type of campaigning."

"We understand that the grassroots enthusiasm this campaign has generated and the over 3,500 volunteers who are canvassing in Iowa this month is threatening to Dick Gephardt," Trippi said.

Sneaking out-of-staters into a caucus in some towns could be difficult, as all participants meet publicly to declare their preferences, leaving strangers vulnerable to exposure. But Murphy said some precincts have hundreds of participants and a fraudulent voter could blend in.