THERE'S NO reason to doubt Sen. Kennedy when he says that personal reasons produced his decision not to run for president in 1984. And his party should thank him for announcing his decision so early. The senator's all-but-Shermanesque statement allows his party and its other candidates the time they need to raise issues and seek support. We think we hear a widespread sigh of relief, and not just from those who dislike Sen. Kennedy or his ideas.

The political case for the senator's candidacy, as Mr. Kennedy said, was still a strong one -- though not perhaps as strong as he thinks. Mr. Kennedy was just reelected to the Senate in Massachusetts with 61 percent of the vote--a large percentage, but one that is lower than those 11 other Democratic senators got in their states this year. He is associated with the idea of expanding the size and scope of government. But, despite the recession and 10.4 percent unemployment, such policies seem to have little following. House Democrats this week went so far as to call for a $5 billion jobs bill. But Sen. Kennedy, campaigning in the less depressed and more inflationary America of 1980, called for a $12 billion jobs bill, as well as national health insurance and wage and price controls. You don't hear much about these policies any more.

Where does Mr. Kennedy's withdrawal leave the Democrats? There are about as many theories as there are potential candidates. You can hear people buzzing on the street that Walter Mondale or John Glenn or someone else is the chief beneficiary. But who knows? The fact is that none of the possible candidates is known, in depth and in detail, by the voters. None is likely to offer exactly Mr. Kennedy's platform, and certainly none can duplicate his personal appeal. He takes a lot of excitement -- dare we say it? -- and charisma with him. But he does not leave an empty field. Mr. Kennedy's vote in 1980 was disproportionately northeastern and black; but none of the major Democrats seems to differ on civil rights, and none claims a panacea for the economic problems of the older industrial states.

The other candidates are busy staking out positions on issues of importance in the 1980s -- trade relations, nuclear nonproliferation, economic production. Now their contest will continue without the special effect of the Kennedy presence or prospect. For them, it is a mixed blessing in a way. They are free of the challenge -- but also will now be subjected to a newly serious and critical scrutiny as men who might just make it. The Kennedy alternative is gone.