THIS YEAR Pat Robertson's Freedom Council plans to spend $4 million to encourage fundamentalist Christians to participate in politics. It has spent more than $300,000 so far on such activities in Michigan, where the Republican precinct delegates who filed to get on the ballot May 27 and elected in August will meet in January 1988 to begin selecting national convention delegates. Much of that money went to a $90,000 rally in which Mr. Robertson spoke to 3,000 followers in Detroit's Cobo Hall and to thousands of others on closed-circuit hookup throughout the state. This is big money in presidential politics: the leading Democrats in 1984 spent about $10 million on prenomination campaigning. And it has been money well spent so far: the Freedom Council filed 4,900 of 9,600 Republican precinct delegates in Michigan and 600 Democratic precinct delegates besides.

To some people, these facts raise the uncomfortable question of whether the Freedom Council is operating according to law. It was set up to be a tax-exempt foundation, and it has accept substantial donations from Mr. Robertson's tax-exempt Christian Broadcasting Network -- $250,000 a month according to a Freedom Council spokesman.

A case can be made that the Freedom Council is entitled to tax-exempt status. Voter registration campaigns, even if they target their efforts at groups whose voting preferences are well known, can be conducted by tax-exempt groups. The idea is that they are just encouraging people to participate. So are we, says the Freedom Council. But much of its work is targeted narrowly on the presidential selection process. Other potential candidates set up tax-exempt foundations too. Some are engaged in work that is plainly nonpolitical (Bob Dole's foundation helps the handicapped), while others run by Gary Hart, Jack Kemp and Bruce Babbitt study issues of public policy. Somewhere between the clearly nonpolitical work of Mr. Dole's foundation and obviously political spending, such as buying a candidate 30-second spots, there is a line.

The decision of whether Mr. Robertson's organization has crossed it was made on May 28 by the Internal Revenue Service when it granted the council tax-exempt status. But even though the question of the Freedom Council's tax-exempt status has apparently been resolved, the public is entitled, we think, to know who has been giving it large sums. Tax-exempt foundations don't have to disclose donors and can accept contributions political campaigns can't. The Freedom Council has been getting contributions as large as $20,000 per couple for a dinner in Detroit. Other potential candidates have disclosed contributions to foundations and PACs they control, even when they're not required to. Mr. Robertson should too.