A blue-ribbon commission appointed by President Reagan to assess the U.S. nerve gas arsenal is stacked with supporters of the administration's modernization plans and pays consultants $250 a day to help promote its recommendation for new chemical weapons, according to commission sources and testimony in Congress yesterday.

Commission Chairman Walter J. Stoessel Jr., testifying before the Senate Foreign Relations Committee, said the presidential panel unanimously supports modernization of the U.S. stockpile, which the commission found to be obsolete, hazardous to handle and useless as a deterrent against a Soviet chemical attack.

Reagan appointed the commission six weeks ago at the request of Congress, which asked for an independent analysis of the administration's proposal to end a 16-year moratorium on the production of chemical weapons.

At yesterday's hearing, however, the panel's recommendations were overshadowed by challenges to its credibility. The attack was led by opponents of Reagan's $174 million modernization proposal who questioned whether the panel was rigged to support the president's bid.

"The product of this commission is not going to inspire a lot of confidence . . . ," said Sen. Carl Levin (D-Mich.).

Stoessel, a former ambassador who served as undersecretary of state from 1981 to 1982, acknowledged under questioning that before the commission was formed, three of the eight panel members -- former secretary of state Alexander M. Haig Jr., former representative Barber B. Conable Jr. (R-N.Y.) and former representative John N. Erlenborn (R-Ill.) -- favored modernization of the stockpile.

Another commission member, John G. Kester, former special assistant to the defense secretary, also has supported a modernized arsenal.

No modernization opponents were appointed to the commission, Stoessel said, despite a request by members of Congress to include former representative Ed Bethune (R-Ark.), who helped defeat administration modernization proposals in the past three years.

The commission is expected to issue its final report next month. Its executive director, Thomas J. Welch, is deputy assistant defense secretary and the administration's leading spokesman for chemical weapons. Its staff director is Col. Warren Mullins of the chemical research and development center at Edgewood Arsenal.

Asked about the commission's objectivity, Stoessel said members "came to the task with an open mind. We heard pros and cons."

Stoessel also was asked about the commission's hiring of a public relations specialist from the Hannaford Co. Inc., the former firm of Michael K. Deaver, Reagan's deputy chief of staff.

When Stoessel replied that the specialist was hired to handle media coverage and contacts between the commission and reporters, Sen. Edward M. Kennedy (D-Mass.) called it an extraordinary move intended to "promulgate the commission's recommendations."

Commission officials said later that Joseph Norton, a Hannaford vice president, earns about $250 a day for his work. The commission also pays Hannaford senior consultant John A.C. Gibson $260 a day to be its congressional liaison.

Gibson, a former Senate staffer and lobbyist, has advised the panel how to promote its recommendations to Congress, a commission source said. The source said Gibson was initially directed to lobby for new chemical weapons but later was told to restrict his activities to liaison with legislators and their staffs.

"This commission was appointed by the administration to get additional justification for new weapons," the source said. "It was very clear that was the point of this exercise."

Welch said that past presidential commissions have hired consultants to handle relations with the media and Congress and that the chemical weapons panel was following precedent.