ARLINGTON, TEX., JAN. 9 -- Senate Minority Leader Robert J. Dole (R-Kan.) sought today to play down an episode involving apparently fraudulent petitions for his campaign here as he stepped up his attack on Vice President Bush's "credibility problem" in the Iran-contra affair.

Dole and his national campaign chairman, William E. Brock, spoke at a news conference here following Dole's appearance before a meeting of the Texas Conservative Leadership Conference.

Both said that despite the apparent forgeries on some petitions circulated in the Houston area, Dole's Texas campaign had collected more than the necessary 5,000 legitimate signatures of registered voters to have his name placed on the ballot in this state's March 8 primary, the largest of the "Super Tuesday" contests.

"I don't think I should be penalized if someone is out there ripping us off," Dole said.

"It's about a two- or three-day story," he added. "It doesn't involve the candidate. I'm the candidate. I didn't run around forging people's names."

Dole then responded to Bush's assertion during Friday's debate in Des Moines that he had answered all questions about his role in the Iran-contra affair except those involving his private advice to President Reagan.

"If George Bush really wants to put to rest all the Iran-contra thing, he should release all his notes, all the memoranda that have come to him from agencies except those directly dealing with the national security and involving the president," he said.

The news conference, however, was dominated by the fraudulent-petitions incident, which has embarrassed the Dole campaign and is under scrutiny by the Federal Bureau of Investigation and U.S. prosecutors.

The investigation also involves apparently fraudulent signatures on petitions circulated by the campaign of Alexander M. Haig Jr. Late Friday, Texas Republican Party officials asked the investigators also to look at the petitions of a third GOP candidate, former Delaware governor Pierre S. (Pete) du Pont IV.

Aides to du Pont said today that they are confident they have enough legitimate signatures to qualify for a place on the ballot.

Texas GOP Chairman George Strake, however, told United Press International that Haig and du Pont may lose their spots on the ballot, although Dole appears to be all right. "Just based on numbers, I would say that Haig and du Pont have some problem," Strake said in Houston.

Brock reiterated today that the fraudulent signatures apparently resulted from the work of a political consulting firm and a subcontractor that the Dole and Haig campaigns hired to help in the petition drive.

But Brock, who said Dole campaign officials "really don't understand why or how" the forgeries took place, said he takes responsibility for the episode.

"I really don't see this as affecting us in any fundamental way," he said. "It's a lousy story. It shows we did not have the oversight we should have had. That's my failing."

Brock, a former senator from Tennessee and labor secretary in the Reagan administration, took over the chairmanship of the Dole campaign in November. Since then, he has made at least one trip out of the country on private business and spent nine days on a Caribbean island with his family during the Christmas holidays.

Asked today whether he is paying close enough attention to the Dole campaign, Brock said:

"Yeah. I think we're doing well. I committed to my children to take them on that trip six months before I consented to this campaign . . . . You're damn right. I'm busting my rear. I'm very comfortable with where we are."

Dole and former television evangelist Pat Robertson were the only Republican presidential candidates to appear at the conservatives' conference, where Robertson received by far the most enthusiastic reception from about 800 people. Du Pont and Rep. Jack Kemp (R-N.Y.) addressed the group by telephone.

Robertson won a straw poll of conference participants with 46 percent of the vote. Kemp finished second with 28 percent, followed by Dole with 11 percent, du Pont 9 percent, Bush 3 percent and Haig one percent.

Bush, who claims Texas as his home state, angered leaders of the coalition of conservative groups by refusing to appear or speak by telephone.

"He could call from anywhere in the world," said Richard Ford, chairman of the conference. "I consider it an insult and very bad judgment. That's not the way Texans treat other Texans."