QUITE A FEW savvy folks, not all of them found in the Democratic Party or in this city, seemed simply to assume -- until the morning after -- that some earlier Act of Tenure authorized Democratic control of Congress in perpetuity. The returns from last Tuesday's Senate contests repealed at least one-half of that myth. The other half is being made ready for assault by the resurgent Republicans, who are contemplating the census-required shift of House seats from the Democratc Northeast to the more Republican-minded Sun Belt. Next come the 1982 congressional elections. And somewhere in between now and then, if the Democrats are serious about recapturing support, the party will have to find a new or at least a greatly improved message.

Many of the Democrats' problems were evident before this election, and many of them even predated the national campaigns of Jimmy Carter, although to hear it from Democrats these last few days is not to understand that.

We will leave it for others to determine when and how an election landslide becomes a popular mandate. But judged against the state-by-state totals of the presidential voting, the immediate Democratic prospects seem to be located somewhere between slim and none. Even allowing for the challenge of John Anderson, it is a fact that this incumbent Democratic president won a smaller percentage of the popular vote this time in Alaska, Arizona, California, Colorado, Idaho, Iowa, Montana, Nebraska, Nevada, North and South Dakota, Oregon, Utah and Wyoming than Sen. George McGovern did in 1972. That amounts to an entire area of the country in which the Democratic ticket was non-competitive.

Part of the party's present plight is directly attributable to superior Republican effort. The Republicans outspent, outorganized and outworked the Democrats across the country. The Democrats over the past decade, in terms of mechanics like candidate recruitment and technical assistance, have unilaterally disarmed in the face of the Republicans' challenge. But more important even than the mechanics is the message in politics -- and the Democrats' present message for many voters is neither salient nor selling.

This is both a success and failure story in its way. At times, certain Democrats hae seemed unwilling or unable to accept and to acknowledge the consequences of successes under previous Democratic administrations -- running, instead, against old and long-since-banished demons. They also were to some extent victimized by success: people whose concerns had changed with the times and who had been freed from some of the afflictions the Democrats had banished didn't need the poor old Democrats anymore.

Yet as important as accommodating to certain successes is, acknowledging failures is just as important. And here the losing party clearly needs to reconsider some of the obsolete and wrongheaded policies it still clings to. Quick-fix dollar programs are not producing good enough results -- and people being pinched by inflation are tired of paying for it all anyway. Democratic leaders (whoever they are these days) are going to have to ask themselves hard questions about how to cope with such issues as the seminal event of this generation -- doubt-digit inflation.

On the opposite page today, Democratic Sen. Carl Levin of Michigan discusses some of his party's defaults and its mistaken persistence in certain programs that were hurting more than helping their intended beneficiaries. We recommend this piece to you. It is honest and constructive, typical of the kind of self-criticism that will be required for any Democratic return from the dead. In the course of the next few weeks and months, there will inevitably be a certain amount of brooding and contending over who gets to speak for the party among its survivors in the statehouses, at the national committee and on the Hill. It will be too bad if, in the excitement over this, the need for the Democrats to rehabilitate their message itself gets lost.