WHEN IT COMES to presidential contests, Michigan Democrats obviously know how to put on a real cliffhanger.

Four years ago, then Democratic front-runner Jimmy Carter and his most persistent tormentor of that season, Arizona Rep. Morris Udall, went down to the wire, with Mr. Carter escaping defeat by a 1,820-vote margin. In a state with 7,008 precincts, that qualifies as a razor-thin margin.

Last Saturday, President Jimmy Carter squared off in Michigan against his relentless challenger of this campaign year, Sen. Edward Kennedy. Again the race was close. Sen. Kennedy barely edged the president by 234 raw votes.

We should mention that there were a few significant differences between the two elections. In 1976, the choice was made by 703,702 voters; in 1980, only 16,048 Michigan Democrats were involved in the selection of 141 national convention delegates.

That's right: only 16,000, or three-tenths of 1 percent of the registered voters of Michigan. Thirty-nine percent of all eligible Democrats did in fact show up at their designated caucus locations on Saturday. The reason that only 41,717 citizens in the entire state were even eligible to participate is a story in itself -- a story of political reform.

Successive reform commissions of the national Democratic Party have outlawed all sorts of nefarious activities like proxy voting, the despised unit rule and closed slate-making. Commissions have also been rather pushy on the subject of Democratic convention delegates being chosen only by Democrats. No Republican or independents or Whigs need apply.

The state of Michigan, following the example of neighboring Wisconsin, has sponsored an open primary -- open to all citizens regardless of creed, color or political persuasion. The Democrats of Michigan were dutifully opposed to such a system, which had been clearly outlawed by the party's reform commissions, and tried unsuccessfully to change the state law.

So Michigan Democrats devised a plan to limit the delegate-selection process to enrolled Democrats. Because the state would not enroll voters by party, the Michigan Democratic Party took it upon itself to do so. The problem was that the party was able to register only a few more than 40,000 of the state's more than five million voters before registration closed on Feb. 26.

But the mandate of the reform commissions had been honored. Republicans and independents were completely excluded from last Saturday's delegate-selection caucuses in Michigan. The principal problem that remained was that the reformed system also excluded almost all the state's Democratic voters as well. But there is always some small price we have to be willing to pay for improvement.