After a two-year lobbying effort by aerospace companies, the National Aeronautics and Space Administration agreed late last year to consider ending Morton Thiokol Inc.'s monopoly on manufacturing the space shuttle's solid-fuel rocket boosters.

A possible fuel leak in the shuttle Challenger's right-hand booster has been a focus of NASA's investigation into the explosion that destroyed the shuttle on liftoff last week and killed its seven crew members.

In a news release Dec. 26, NASA said it would look into opening the $1.5 billion contract to competitive bidding, citing "a broadening of the rocket booster industrial base."

The decision followed an industry lobbying effort on Capitol Hill described by congressional staffers as "a full-court press."

More than 80 House members signed a letter last August asking space agency officials to consider allowing another company to bid, and in interviews this week, key members of the House Science and Technology Committee have questioned whether lack of competition may have compromised the boosters' quality.

The contract, one of the largest in the space industry, was awarded to Thiokol in 1973. A NASA spokesman said that it covers the first 38 shuttle flights and that the company is negotiating to provide boosters for the next 60 flights.

Last week's Challenger mission was to have been the 25th shuttle flight.

"Once the period of mourning is over, we are going to have to look at who is to blame," said Rep. Robert G. Torricelli (D-N.J.), a committee member and chief sponsor of a bill to build a new shuttle. "Many of us have questions about the reliability of Morton Thiokol. Without competition, they have had enormous profits and little motivation."

Gilbert Moore, a spokesman for the Chicago-based company, said Thiokol rockets launched 24 shuttle flights "that performed just fine, and that sounds safe." He declined to comment on specific criticisms, saying Thiokol "is concentrating its entire resources on helping NASA with its investigation."

Torricelli said representatives of Hercules Inc., a Wilmington, Del., aerospace company that wants to bid on the contract, have told him that Morton Thiokol "used outdated technology and dangerous facilities." He said he took Hercules board chairman Alexander Giacco to press his case in a private meeting with then-NASA administrator James Beggs last year.

Thiokol manufactures the boosters at its Brigham City, Utah, plant. Torricelli said a $3 million explosion and fire there last June, touched off by lightning, raised questions about whether a single company could maintain an adequate supply of rockets to ship to its assembly facility at the Kennedy Space Center.

Rep. Norman Y. Mineta (D-Calif.), another key member of the subcommittee that oversees NASA, said last week's accident will result in closer oversight.

"NASA has not been asked the hard questions in the past because of the mystique of the agency, the glamor . . . , " he said. "The Congress ought to look at what are the systemic problems that allowed this to occur; if it is morale, discipline or training, why wasn't it dealt with last year or the year before? Why haven't we heard about it before?"

A report released Monday by NASA to the news media cited lack of discipline, vigilance and training on the part of Thiokol workers involved in a booster-assembly accident at Cape Canaveral. The damaged booster was never used.

Besides Hercules, three other companies have lobbied to bid on the booster contract: United Technologies Corp. of San Jose, Calif., Aerojet Strategic Propulsion Co. of Sacramento, Calif., and Atlantic Research Corp. of Alexandria, Va.

Henry A. Schowengerdt, president of Hercules Aerospace Corp., said that while "competition sharpens you up on everything -- cost and quality -- I wouldn't say that Morton Thiokol is characterized by complacency." He said Hercules and Thiokol operate jointly on some government contracts.

But Schowengerdt said NASA is "living dangerously in terms of their industrial base. They have all their eggs in one basket."