SAY WHAT YOU will about him, Ronald Reagan has done a great thing for '50s liberals, that much reviled group of stodgies and recalcitrants who never got quite right with the 1960s, tending as they did to keep their clothes on in public places and periodically to betray a certain vestigial respect for authority. We know whereof we speak, since, to be blunt about it, it is often the voice and values associated with '50s liberals to which you are exposed in this space. And what has Mr. Reagan done for the holders of these antiquated views, keepers of the dreary, moderate-middle mildly-leftish flame? Quite simply, he has made us feel young again.

There we were, after all, in the rear guard of social and political action for the past two decades, trying vainly to explain that there had been a time when our subsequently accepted, enacted and seemingly ho-hum ideas had been, well, quite advanced. Now, thanks to the president, they are advanced again. Not just advanced, but downright radical. Federal aid for the poor, more generous welfare benefits, relief for the cities--suddenly, without having to do a thing, we are right back out there on the cutting edge of progress, involuntarily re-created by Mr. Reagan as the dangerous social agitators we used to be. It is heady, terrific stuff. If we may slightly recast Wm. Wordsworth's hymn to his political youth in the days of the French Revolution to characterize our own developing feelings about the Age of Reagan, "Bliss was it in that dawn to be alive / But to be middle-aged was very Heaven!"

Heaven, that is, for well-off, superannuated '50s liberals, looking for a trip down memory lane. But not so heavenly, perhaps, for the people who were meant to be helped by the earlier legislation that was argued for in the '50s and passed in Lyndon Johnson's day--people who were going to be dealt with more generously and fairly and to be made at least minimally secure by those long since accepted federal statutes and commitments.

If there was a single theme to all the legislative history the Reagan administration now seems eager to undo it was this: that there were (are) certain afflicted, down and out or disadvantaged groups within the society whose troubles have their source in an array of national, not local, circumstances and for whose improved welfare there is, correspondingly, a national responsibility. Economic decisions made by the feds to help one group may have hurt another. Black people migrating from a segregated South in search of a better economic break end up in overcrowded economically depressed northern manufacturing cities. These destinies are all interrelated. They are not some single jurisdiction's "problem." There was an evolving national consensus that the federal government, acting on a new-found sense of national community and of community obligation was the proper instrument to deal with these problems. Is that consensus gone? Gov. Bruce Babbitt of Arizona raises the same question with some vigor on the opposite page today.

We are aware of the general idea of a "swap." But it is also true that the programs Mr. Reagan would turn back to the states are those proffering help to the groups most at risk and most vulnerable to the indifference or even hostility of state legislatures and pressed local taxpayers. It was a great and overdue moral--yes, moral--step a couple of decades ago when the country accepted responsibility for the well-being of these citizens. And it might be noted that the term "turn back" is itself hardly apt, since these federal programs tended to get started precisely because the states and localities refused to act. So one huge question about Mr. Reagan's swap idea concerns whether he is content merely to swap the poor and the discriminated-against and various other victims in the society back to a worse condition.

In that sense, anyway, the 1950s deserve to stay firmly in the past. A little political nostalgia can go along way.