The Federal Election Commission ruled yesterday that Rep. Jim McCrery (R-La.) can use campaign funds to pay for a babysitter while his wife joins him at "vital" events such as receptions, cocktail parties and news conferences.

The commission issued its opinion in a unanimous vote. FEC staff had recommended rejecting a Nov. 1 request by McCrery, a fifth-term congressman from Shreveport. He and his wife Johnette have an infant son, Scott, and are expecting a second child in March.

Federal election law prohibits converting campaign funds to "personal use," defined as any expense that "would exist irrespective of the candidate's campaign or duties as a federal officeholder."

Federal election law expert Kenneth Gross said the commission changed its regulations on personal use of campaign funds last year to limit the things on which campaign money could be spent.

Earlier, the FEC had allowed the use of campaign funds for such items as purchase of tuxedos and evening gowns, tuition, country club dues, salary payments to candidates, tickets to sporting events and funeral expenses. In pressing his case, McCrery wrote in November that "Johnette often accompanies me to campaign-related events such as receptions, cocktail parties, press conferences, finance meetings, etc., which are inappropriate for our 20-month-old son."

Furthermore, McCrery added, his "large district" stretches across northern Louisiana, requiring "us to travel extensively at various times, often including an overnight stay." Child care would also "occasionally" be necessary when this occurred.

In all cases, he wrote, "Johnette is an integral part of my campaign team, and her presence at these events is vital." Thus, the baby-sitting expense "is directly related to my candidacy and would not exist" if he wasn't campaigning.

In December, however, the commission staff decided that McCrery's problems were no different from anyone else's. "Providing for child care when a parent is unavailable for business travel reasons is a concern for most families, regardless of profession," the staff said in its draft opinion. "The expenses for such care cannot be said to be specially related to your campaign."

According to commission spokesman Ron Harris, however, this view did not win approval when commissioners met Dec. 14, and no ruling was issued.

Commissioner Scott E. Thomas later proposed in a memo that McCrery's request be approved as long as the commission made clear that its ruling was a "bare-bones" response that dealt specifically with the McCrery case without opening the way for a barrage of child-care petitions.

"I do not think that we are starting a parade of horribles,' " Thomas wrote. But he added, "We will no doubt have some difficult advisory opinion requests in the future." He posed several hypothetical questions:

"What if the need for child care arises virtually all day every day for several months?" he asked. "What if the child care relates to an event held in the candidate's home? Does child care extend to boarding school tuition?"

Nonetheless, he continued, "I'd prefer to give the right answer to each than the wrong answer to any one of them."

The commission endorsed Thomas's substitute language approving McCrery's request "in the limited circumstances you present." The opinion noted that the McCrerys "occasionally" needed to attend events together, and that "child-care expenses will be incurred only as a direct result of campaign activity."

McCrery was traveling in his district and could not be reached for comment. Staff writer Ruth Marcus contributed to this report. CAPTION: REP. JIM McCRERY