HOWARD BAKER, lagging behind other possible Republican presidential candidates in fund raising, has raised an issue that may reduce their financial advantage. Mr. Baker, like George Bush, Jack Kemp and Bob Dole, has a political action committee that has been paying for most of his politicking around the country. But he doesn't think PACs should be used to finance presidential campaigns. He has appealed to the Federal Election Commission for support, and has apparently got it.

Last week the FEC issued a tentative advisory opinion saying a candidate can't use more than $5,000 in PAC money to pay for what amounts to presidential campaigning. It said such spending is an exploratory committee expense. Once a candidacy becomes official, exploratory expenses become officia presidential campaign expenses, toward which a PAC can legally give only $5,000.

Mr. Baker obviously hopes this opinion will force George Bush, Jack Kemp and Bob Dole to shut down their PACs and set up exploratory committees -- Mr. Bush's especially, since it has field staff on the ground in Michigan and New Hampshire. Bush and Kemp partisans reply that they're careful not to talk about 1988. They may seek advisory opinions from the FEC based on their own factual situations. The distinctions here can get pretty fine. Do you have to post signs in hospitality suites saying "Don't think about '88"?

The line drawn in the FEC's Baker advisory between presidential campaigning and other political activity may produce some arbitrary distinctions, but any line would, and this one seems to us as good as any. The other candidates may go on for a while using PACs to finance candidate travel. We think they would be better advised to pay for their exploratory and campaigning activity through exploratory and campign committees.

We must add, however, that the Baker opinion does have two perverse effects. First, it tends to make the campaign begin even earlier. It begins early anyway, but there is an argument -- perhaps Miss Manners would be the one to advance it -- that a little hypocrisy sometimes makes political life more bearable. And a sitting vice president may have good nonpolitical reasons for not wanting to declare.

The second perverse effect is that, if you force the candidates' PACs out of business and make them set up exploratory committees, you are, for a moment, taking things out of public light and casting them into darkness. PAC contributions and expenses have to be disclosed at regular intervals; exploratory committee contributions and expenses don't have to be disclosed until (and if) the candidacy becomes official. But that is an argument finally for telling it like it is, making official announcements and setting up campaign committees. If you're really running, why not say so?