Supporters of Ronald Reagan yesterday formed a Reagan-for-President Committee, claiming the former California governor now represents the mainstream of American political thought.

In announcing the committee, Sen. Paul Laxalt (R-Nev.), said Reagan, long the darling of the conservative wing of the Republican Party, will run not as a "fringe" candidate for president in 1980, but as a middle-of-the-roader.

Reagan hasn't changed, according to Laxalt and other Reagan advisers, but the country has. "Not since Gen. Eisenhower's first election almost 30 years ago has there been such a perfect 'fit' between the man and the public mood as there is today with Gov. Reagan and the American people," Laxalt said.

The makeup of the committee was designed to show Reagan's early muscle in the presidential sweepstakes. Its 365 members are drawn from every region of the country. They include the nucleus of the 1976 campaign organization, five U.S. senators, 22 House members and a host of prominent supporters of his former rival, then-President Ford.

Among the former Ford backers endorsing Reagan were former treasury secretary William Simon, former agriculture secreatary Earl Butz, former health, education and welfare secretary Caspar Weinberger, former interior secretary Stanley Hathaway, and former West Virginia governor Arch Moore.

The leadership of the committee, however, will be in the hands of time-tested Reagan loyalists. Laxalt, chairman of the Reagan committee, held a similar position four years ago. John P. Sears, committee executive vice chairman, is a longtime Reagan strategist. Deputy Chairman Michael K. Deaver, who will oversee general operations, and Franklyn Nofziger, the fund-raising head, have worked with Reagan since his days as governor.

The committee has Reagan's official approval, but its formation does not represent a declaration of candidacy, Laxalt said. But when asked if there was any doubt in his mind that Reagan will be a candidate, Laxalt said, "No."

The committee will enable supporters of Reagan, who isn't expected to formally announce until next fall, to raise and spend campaign money and form state campaign organizations.

The committee, a spokesmen said, will also allow Reagan to continue to market his lucrative radio commentary show and newspaper columns without violating Federal Communications Commission equal time rules. Reagan, he said, will also continue to accept fees for appearing before business groups, although the number of invitations for such appearances is expected to taper off.

Since Ford defeated him for the 1976 Republican nomination, Reagan, 68, has kept his political hopes alive by traveling around the country under the auspices of the Citizens for the Republic, a political action committee he formed with surplus 1976 campaign funds. Spokesmen said the campaign committee aides who have been on the CFTR will not disassociate themselves from it.

The packed room and crowd of dignitaries who sat behind Laxalt at the Capitol Hill Club yesterday provided a marked contrast to the group present in 1975 when Laxalt announced the formation of a similar committee for Reagan. At that time only three Republican leaders endorsed Reagan -- Laxalt, former Iowa congressman H. R. Gross, and former Kentucky governor Louie B. Nunn.

"I was almost by myself," Laxalt said yesterday. "It was a lonely scene."

Although it was protrayed as a broad-based group, the majority of the people on the Reagan committee represent the conservative wing of the party. Senators, for example, are Jake Garn (R-Utah), James A. McClure (R-Idaho), Orrin G. Hatch (R-Utah), and Richard Schweiker (R-Pa.), a one-time moderate whose voting record has become increasingly conservative since Reagan said he'd name him vice president if he won the 1976 nomination. Former Virginia senator William Scott is also a member of the committee. The only current governor on it is Charles Thone of Nebraska.

Reagan, a onetime, movie idol, also was endorsed by a number of former Hollywood associates, including Pat and Debby Boone, James Cagney, Frank Sinatra, James Stewart, Loretta Young, Irene Dunne and Efrem Zimbalist Jr.

Other top Reagan aides announced include Charles Black, national political director; Martin Anderson, director of research; Ed Meese and James H. Lake, senior consultants; Loren A. Smith, general counsel, and May Buchanan, treasurer.