A group of influential Republicans yesterday launched a drive to raise from $20 million to $30 million to promote Ronald Reagan's presidential candidacy by taking advantage of a loophole in federal election law.

If successful, the group, headed by Sen. Harrison Schmitt (R-N.M.), would be in position to make "independent expenditures" almost equal to the $29.4 million in federal campaign funds major party nominees receive and are limited to spending under law.

The group, called Americans for Change, could do so by exploiting a provision in federal election law allowing unlimited spending by committees as long as they have no contact with the campaign.

If this method of financing proliferates, it could undermine election law reforms designed to place limits and controls on federal election spending dating back to the Watergate scandals.

Americans for Change the second independent group to announce plans to boost Reagan's candidacy through independent expenditures. The Fund for a Conservative Majority, a conservative political action committee, made more than $500,000 in such expenditures in Reagan's behalf during the primaries. A third group, made up of former officials of the Nixon and Ford administrations, is considering making a similar effort. It reportedly is talking about raising from $12 million to $15 million for pro-Reagan advertising.

The number of groups, however, raises questions about their ability to meet their multimillion-dollar goals. "They're coming out of the woodwork," said Ralph J. Galliano, executive director of the FCM, which has been making independent expenditures for various candidates since 1976. "Right now everyone is jumping in on this."

No such groups have yet been formed to offer similar help to President Carter, the likely Democratic nominee.

"This is a legal committee," Schmitt said in an interview. "We are not trying to get around the law. The independent expenditure provisions were put into law to provide a way that First Amendment rights can be exercised.

"Without them, there would be a serious limit on the rights of individuals to free speech," he added.

Americans for Change includes a broad cross-section of Republicans on its steering committee. Among them are former defense secretary Melvin R. Laird; Clare Boothe Luce, a former ambassador and congresswoman; Minnesota Sen. Dave Durenberger; former Michigan governor George Romney, a supporter of George Bush's unsuccessful candidacy: former South Carolina governor James Edwards, an early Connally for president supporter: former senators George Murphy of California and Marlow Cook of Kentucky; Washington businesswoman Anna Chennault and William Miller. Sen. Barry Goldwater's vice presidential running mate in 1964.

The officers of the group, which claims to have already raised $150,000, include John Harmer, Reagan's lieutenant governor in California; Brad O'Leary, former executive director of the Texas Republican Party; former senator Carl T. Curtis of Nebraska; and Stan Huckaby, former Baker for president treasurer.

The group plans to raise money by contacting wealthy donors who have already given to Republican presidential candidates and through direct-mail appeals. The funds will be used, Schmitt said, for "communications" -- including television ads, surrogate speaking tours and direct-mail promotional campaigns.