NATIONAL DEMOCRATIC PARTY rules restrict participation in the delegate selection process to Democrats only. That seems reasonable, but Michigan state law says that primary elections shall be open to all voters. So Michigan Democrats won't have a primary. Instead, next Saturday they will select their 141 delegates to the Democratic National Convention in 89 separate caucuses throughout the state.

Michigan did hold a primary in 1972 and, to the enduring embarrassment of the state's liberal Democrats, gave George Wallace 51 percent of the vote. To participate this year in the caucuses, a voter must have been an enrolled member of the Michigan Democratic Party for 60 days preceding the caucuses.

In 1976, over 700,000 Michigan voters cast ballots in the presidential primary, which Jimmy Carter won over Rep. Morris Udall by 1,820 votes. This year, that margin could look like a landslide, because only 41,717 Michiganders (and Michigeese) are enrolled as Democratic Party members and therefore eligible to select convention delegates pledged to either President Carter or Sen. Kennedy.

Michigan Republicans (who will hold a presidential primary on May 20) and some Detroit newspapers have criticized the state's Democrats for diminishing the importance of the primary and for reimposing what they call a modified "poll tax." Michigan Democrats, on the party membership forms they distributed, did what any political party would do given the chance. They provided optional plans for membership, beginning with the $10 basic and going up to the $100 "century" (what else?) category and then down to the $2 special for retirees and the $3 discount for full-time students. There was no special rate, as best can be determined, for retirees who are part-time students. Buried in the corner of the application -- to comply with national party rules, which prohibit the imposition of fees for participation -- was a box to check off if one chose not to pay any dues.

The too-bad part is that only 41,717 people bothered to fill out the forms and select any of the simple or exotic plans at all. But if everyone shows up at his or her caucus on April 26, his (or her) chances of becoming a national convention delegate are better than 1 in 298. That's a lot better than they were in either Iowa or New Hampshire and considerably better than the odds at any gaming table in Atlantic City.