CLEVELAND, JULY 17 -- Democratic presidential candidates demonstrated today that they think the Iran-contra hearings have made President Reagan and his staff tempting political targets.

Five declared and prospective contenders for the nomination turned an afternoon forum organized by the Association of State Democratic Chairmen into a verbal shooting gallery using the people who have filled the nation's television screens during the past two weeks. Three other candidates are to appear on Saturday.

Declared candidates Rep. Richard A. Gephardt (Mo.), Massachusetts Gov. Michael S. Dukakis, Sen. Paul Simon (Ill.) and Bruce Babbitt and prospective contender Rep. Patricia Schroeder (Colo.) took few cracks at each other, but competed in denunciations of the Reagan staff and the man who appointed them.

Babbitt, the former governor of Arizona, said the hearings had revealed "a labyrinth of misdeeds and misrepresentations." If they go on long enough, he joshed, "they may yet establish a link between Ronald Reagan and the presidency."

Gephardt also ridiculed Reagan's "bystander presidency," which he said had allowed "a junta of colonels to take over direction {of policy} from anyone elected by the American people."

Gephardt said he understands American "sympathy" for fired National Security Council aide Oliver L. North, but said, "North sure was wrong with what he did. He put the Constitution of the United States through a shredder."

Simon also criticized North by name. He told the state chairmen that the mail and messages to his office had run 100 to 1 in North's favor, but he had told a convention of county officials last week, "Colonel North is not a hero. There were boos, but I told them our heros are not those who lie, who destroy records, who disobey our laws."

Dukakis said that while he had always thought the rule of law "a very conservative idea, we've got a bunch of conservatives in the White House who wouldn't know the rule of law if it hit them in the face."

In answer to a question, he rejected the idea of requiring Senate confirmation of the president's national security adviser, but promised that if he became president, his appointees' actions would be subject to public scrutiny.

"It's beyond me . . . how anyone thinks you can conduct the nation's business in secret . . . . There is nothing about a privatized foreign policy in the Constitution," he said.

Schroeder, a representative from Denver who has begun exploring a possible candidacy, noted the irony of the nation marking the 200th anniversary of the Constitution while "Ollie North is down in the basement ripping it up."

She said the administration had demonstrated "a contempt for public service . . . . You've seen it with the contra affair . . . an absolute violation of democracy."

Party chairman Paul G. Kirk Jr. said Democrats would make an issue of the behavior publicized in the congressional hearings, what he called the Republican record of "deceit, defiance, diversion and deniability."

Kirk said that pattern extended beyond the Iran-contra affair and the "sleazy greed which has indicted the highest levels of the administration" and included Reagan's rejection of responsibility for the growth of deficits in the last seven years.

The candidates' session was threatened by cancellation in a labor dispute between NBC and the National Association of Broadcast Engineers and Technicians (NABET). Facing the threat of picketing by NABET members if a nonunion crew taped the candidates for NBC and the network-owned local affiliate, the Democratic state chairmen denied credentials to NBC. This morning, NBC obtained a court order requiring the credentials to be issued.

With candidates unwilling to cross a threatened picket line, a pact was negotiated in which NBC agreed to send its crew in through a special entrance, and NABET agreed to picket only that entrance.