When Marine Lt. Col. Oliver L. North, the central figure in the Iran-contra affair, finally testifies this summer, producing perhaps the most riveting television of the year, freshman Sen. Paul S. Trible Jr. (R-Va.) will be one of four members of Senate and House select committees leading the questioning.

Derided as "Plastic Paul" by some Virginia Democratic leaders and written off by some on Capitol Hill as a peripheral player in the complicated case, Trible has surprised many with his dogged interest in detail and willingness to attack the administration at times.

In the five weeks since the committees began jointly probing the secret enterprise reportedly dreamed up by North and others to use profits from Iranian arms sales to provide support to Nicaraguan rebels, Trible has steadily maneuvered his way to the center of the politically sensitive hearings.

To do this, Trible, a conservative Republican who is up for reelection next year, gambled by allying himself with Sens. Warren B. Rudman (R-N.H.) and William S. Cohen (R-Maine), who are among the most aggressive committee members exploring the alleged misdeeds of the Reagan administration and its covert agents. In contrast, other Republicans, including Sen. Orrin G. Hatch (R-Utah) and Rep. Henry J. Hyde (R-Ill.), have used the hearings largely as a forum to defend President Reagan and blame Congress for impeding the administration's Central American policies with vacillating restrictions.

"I think each of us brings to this task somewhat of a different perspective," said Trible, a former assistant U.S. prosecutor and House member. "There are other members of the committee that feel compelled to defend all that's gone on. I have been asking hard questions. I think probing questions are necessary to the pursuit of truth.

"I'm a supporter of Ronald Reagan, and I believe in the contra cause, but I'm deeply offended by what's gone on," Trible said.

While insisting that "there is absolutely no evidence" that Reagan ordered the illegal diversion of arms sales funds to the contras, Trible has reaped increased publicity by focusing on the trail of millions of dollars raised and spent by administration officials and operatives, and by relentlessly hammering at the credibility of Richard V. Secord.

Secord, a retired Air Force major general, arranged the sales of U.S. weapons to Iran in 1985 and 1986 and helped set up the supply operation to the contras at the behest of North, who was then a National Security Council aide. During a testy exchange May 8, Trible told Secord that he was "deeply troubled" by Secord's attempt to ascribe his activities to patriotism when Secord and his partner, Albert A. Hakim, controlled Swiss bank accounts brimming with profits from the sale of arms to Iranians and the contras.

Trible described Secord's associates as "out-and-out profiteers or worse" and questioned Secord's assertion that he had renounced his profits from $11.4 million in arms sales to the contras. Last Thursday, Trible disclosed to reporters that Secord had withdrawn funds from the bank accounts in 1985 and 1986 to purchase a $31,000 Porsche and a $50,000 Piper aircraft and to pay $2,300 for a trip with an associate to a "fat farm."

"Secord's testimony simply didn't ring true from the first day," Trible said. "Secord said he was advancing the national interest. The evidence showed he was actively pursing his own self-interest."

Secord said in an interview yesterday that he had bought the car and airplane, but that Hakim had told him that some of the money came from a consulting fee in a separate business deal and that the rest was a personal loan. Secord said he did not know that Hakim had tapped accounts used for the Iran-contra arms dealings.

"I think {Trible} has been questioning my integrity and motivation for political reasons, which in turn makes me question his motivation," Secord said.

Sven Holmes, general counsel and staff director of the Senate Intelligence Committee, said last week that Trible had "taken hold of some complicated issues, particularly financial issues, and has really dug in."

"Secord's testimony was the first major indication that Trible was taking a serious interest in what's going on," Holmes said. "The people with the best grasp of what's been said are the people who have taken the time to understand the money. Trible is one of those people."

Trible's close relations with Rudman, the vice chairman of the Senate select committee, and with Sen. Daniel K. Inouye (D-Hawaii), the committee chairman, have worked to his advantage. When Trible suggested to Inouye that the members devise a system of dividing responsibility for witnesses and subject areas, Innoye agreed, and he promised every member "equal klieg light time." Trible, the junior member of the select committee, was paired with the more senior Sen. George J. Mitchell (D-Maine), a former federal judge, and the two were designated as the Senate interrogators of North.

Moreover, Rudman shared with Trible some information he had gathered about two federal Drug Enforcement Administration officers who had been assigned to work for North in 1985 as part of a secret year-long effort to use private funds to ransom American hostages in the Middle East. Trible was quoted widely in reports about the DEA operation, including a front-page story in The New York Times May 31.

Columnists Rowland Evans and Robert Novak wrote May 25 that the White House was stunned by the ferocity and animosity of Rudman and puzzled by the "prosecutorial pretensions of Sen. Paul Trible, suppposedly a Virginia conservative and up for reelection next year."

Trible said he had received no complaint or pressure from the White House since the hearings began. "My role is not to indict or defend but to pursue the truth as fairly as possible," he said.

His appointment to the 11-member Senate select committee in December was viewed by many as a mixed blessing for Trible, who had labored for four years in the shadow of Virginia's senior senator, Republican John W. Warner.

While acknowledging the attention that the assignment would generate for the freshman, some Republicans feared that it could backfire if Trible was perceived as overly eager in defending Reagan -- who campaigned for Trible in 1982 -- or overly critical, angering conservatives. "He's walking a tightrope," said Donald W. Huffman, chairman of the Virginia Republican Party.

Trible has been able to capitalize at home on the assignment, drawing generally favorable coverage from Virginia television stations and supportive editorials from newspapers across the state.

"I've watched clips of {the hearings}, but I haven't been glued to it," said Larry Framme, state Democratic Party chairman. "I haven't found it to be very exciting. But politically, yeah, I think {Trible} will get some mileage out of it. How much depends on how much the media makes of him as a performer."

Trible owes his climb to prominence largely to then-Senate Majority Leader Robert J. Dole (R-Kan.), who appointed Trible to the Senate select committee.

During the weekend, Trible endorsed Dole for president at a Virginia Republican workshop in Richmond organized by Trible. Trible insisted that the endorsement had nothing to do with his appointment to the high-profile select committee.

"Bob Dole has been my leader in the Senate and has been a man of great intellect and ability, and he knows how to get results in Washington," Trible said.

While criticizing the White House for arrogantly disregarding the concerns of Congress and "privatizing" foreign policy, Trible has stepped gingerly in areas that would suggest possible presidential complicity in unlawful acts.

"I believe the president when he says he didn't know about the transfer of the diversion monies . . . and I will continue to believe him until proven otherwise," Trible said.

" . . . But nevertheless, it's hard to justify the sweep of these activities from Iran to Central America. The contras were not well served. Our antiterrorism strategy is a shambles, and the ability of Ronald Reagan to be a strong and decisive and successful president has been diminished by all of this."

Trible received a flurry of constituent phone calls and letters complaining about his early clashes with Secord, but Trible said the complaints diminished as more evidence came out. He said he anticipates a similar reaction when the colorful North testifies. There has been testimony that Hakim and Secord set up a $200,000 Swiss bank account as a "death benefit" for North's family.

"Oliver North will not be an easy witness," Trible said. "He's a man viewed very sympathetically by many. He's a man who lives in a world somewhere between reality and fantasy. But he's the most important figure in all these events. And his story must be told, fully and accurately."