The White House had decided to expand the membership of a new presidential study commission on antitrust in order to settle a sqabble among Senate Republicans, administration sources said yesterday.

Officials confirmed that awaiting President Carter's signature is a new executive order expanding by six the 15-member commission he created last December. The exercise is designed to keep Senate minority leader Howard Baker (R-Tenn.) from having to choose between Sen. Jacob Javits (R-N.Y.) and Sen. Strom Thurmond (R-S.C.). Both wanted to be on the commission, but there was place for only one of them.

The primary mandate of the presidential commission is to study and make recommendations on ways to expedite complex antitrust cases. Membership of the original panel was to include John Senefield, assistant attorney general in charge of the Antitrust Division; Federal Trade Commission Chairman Michael Pertschuk; a chairman of an independent regulatory agency, expected to be civil Aeronoutics Board Chairman Alfred E. Kahn; three members each of the Senate and the House; to be selected by those bodies; a federal district judge, and five at-large members.

Of the three Senate members, two would be Democrats - Antitrust and Monopoly Subcommittee Chairman Sen. Edward M. Kennedy (D-Mass.) and Howard M. Metzenbaum (D-Ohio) - and one would be a Republican.

Because of the battle between Javits and Thurmond, Baker was said to have asked the White House to increase the membership in order to accommodate both. The result is a new executive order boosting the membership by two senators, two House members and two from the public. [TEXT OMITTED FROM SOURCE]

The commission is to have a six-month life, but it doesn't begin formally until the last of its members is appointed. A list of proposed members does not accompany the new executive order, sources said.

On Capitol Hill yesterday, members of the Senate Antitrust and Monopoly Subcommittee, led by Kennedy, prodded Shenefield on the antitrust division's conduct of its major cases as well as its failure so far to make good on a number of the administration's promises on antitrust matters over the past year.

In repeated questioning, Kennedy asked whether the division wasn't pursuing too many price-fixing cases, why it wasn't bringing more cases challenging mergers and monopolies, when it would produce the promised energy antiturst policy, and why deadlines appeared to keep slipping.

After rereading administration testimony of a year ago about the importance of drawing up an energy antitrust policy, Kennedy asked, "Where is it? . . . What's the problem?"

When Shenefield replied several times that he had underestimated the time, staff and resources it would take to accomplish some tasks, Kennedy pledged to "go to bat" for a division request for resources and staff beyond the authorization request by the administration if Shenefield tells hin what he needs.