Alice in Budgetland is reversing the logic of that Alice-in-Wonderland line: ''Sentence first - verdict afterwards.'' We mean Alice Rivlin, of course, of the Congressional Budget Office, which was asked for an analysis of President Reagan's budget plan by the Senate Appropriations Committee. She returned with a sobering verdict, leaving the sentencing to Congress.
The legislators ought to be spurred to new urgency in deciding what degree of outright encouragement, probation, or rehabilitation will do the budget the most good. Surely this would be preferable to the willingness of some critics or supporters to sentence the plan to oblivion or set it scot free before a thoughtful verdict has been reached.
Mrs. Rivlin's verdict was that the budget deficit would have been almost erased by 1985 if Congress had not passed last year's tax-cut package. But deficits would be far higher than the Office of Management and Budget estimates based on less spending and more revenues than forecast by the congressional analysts. And deficits would continue to grow - rather than decline, as the administration says - because of cutting taxes and then keeping them down through indexation.
The Rivlin figures underscore Wall Street doubts about the OMB budget figures. These doubts have been stressed by no less than James Schlesinger, who was acting budget director during the Nixon administration. He laments the loss of respect he believes the ''preposterous'' projections have brought on the OMB. The Congressional Budget Office, he wrote last month, ''may well enjoy the satisfaction of providing the only credible official estimates in town.''
Mrs. Rivlin's ''very grim outlook'' is not the last word, to be sure. Present budget director David Stockman has had ready answers in support of his projections. He can cite a number of OMB forecasts for 1981 that turned out close to the mark. But the fact that official figures for the future can disagree so fantastically should propel Congress out of the wonderland of addressing them through politics as usual. Both it and the administration must come back down to earth where you only get what you pay for.