The legal staff of the Federal Election Commission has issued a tough interpretation of campaign finance laws that would appear to restrict severely the use of special political action committees (PACs) as backdoor mechanisms to pay for unannounced presidential campaigns.

The rules, if approved next week by the full commission, would provide grounds for challenging the use of "multicandidate" PACs by such prospective presidential candidates as Vice President Bush, Rep. Richard A. Gephardt (D-Mo.), Rep. Jack Kemp (R-N.Y.) and Sen. Robert J. Dole (R-Kan.).

Multicandidate PACs are those formed to help more than one candidate. In Bush's case, for example, the PAC is raising and spending money on behalf of Republican candidates, particularly those running for the Senate.

Public-interest groups and aides to former Senate majority leader Howard H. Baker Jr. (R-Tenn.) have charged that potential presidential candidates are using these PACs to raise and spend money not covered by the contribution and spending limits applicable to a full-fledged presidential campaign committee.

The proposed rules, contained in an advisory opinion issued in response to a series of questions by Baker, would restrict or prohibit, under certain circumstances, the use of the special PACs to pay travel costs, the expenses and salaries of consultants and supporters, and fees for hospitality suites.

Jan Baran, lawyer for Bush's $3.8 million PAC, the Fund for America's Future, rejected suggestions that the opinion of the FEC's legal staff could be used to challenge expenditures by the Bush PAC. "I don't think this is going to change things for us," he said.

Bush has used his fund to hire a staff of 20, including two full-time aides to work in Michigan and Iowa, key states in the presidential-selection process. He will probably assign a staffer to New Hampshire, another critical primary state, according to Bill Phillips, the fund's executive director. In addition, the fund has been used to pay travel costs for Bush as well as for some of his aides and supporters.

"In Michigan, we are active in encouraging people to get involved at the precinct level," Phillips said. Later this year, Michigan will select precinct delegates who will be critical in selecting the state's delegates to the 1988 GOP national convention.

While having raised less money than Bush, other prospective presidential candidates have used their multicandidate PACs for travel, staff and other costs that may be covered under sections of the FEC legal opinion.

Bill Romjue, director of Gephardt's PAC, justified payment for the cost of travel for speeches by Gephardt on the grounds that raising pro-Democratic issues across the country "leads to a climate that helps all [Democratic] candidates for the House and Senate."

The FEC opinion by Charles N. Steele, chief counsel, declares that travel costs of a prospective candidate who refers to his "potential" candidacy at party gatherings known as presidential "cattle shows" should not be paid by a "multicandidate" committee.

In addition, the opinion says similar prohibitions should apply to charges for hospitality suites for "party dignitaries and press representatives" attending such cattle shows, payment of travel costs to such events by supporters, travel costs of the candidate to discuss his potential candidacy in private meetings, the costs of organizing steering committees made up of potential supporters and the salaries of aides while they promote their boss' presidential prospects.

These costs, according to the opinion, should be paid either by a presidential camapign committee or an exploratory committee, each of which must abide by the tight contribution and spending limits required of presidential campaigns. The most a multicandidate PAC can contribute to these costs is $5,000.