JUXTAPOSED IN this newspaper last weekend were David S. Broder's reports on the exuberant array of Republican officeholders who gathered for their annual Tidewater Conference and Haynes Johnson's articles on the state of Democratic Party disarray. The Tidewater assemblage voted a predictable set of endorsements of administration policies -- with one glaring exception, to which we will return.

From the Democratic notables there emerged mainly prophecies of gloom. To twist the familiar Mark Twain comment, reports of the party's death are -- if anything -- greatly understated. Conservatives laced into what they believed were voter-alienating policies pursued by party liberals, while the latter bemoaned the growing strength of party conservatives. On every side, those interviewed conceded the absence of major new ideas in their ranks as well as a dismal set of demographic and economic trends tending to complete rapidly the shift in national political power to the Republican West and southwest.

Sen. Daniel Patrick Moynibhan prophesied: "We're going to be a party of the Northeast, which is a minority party by definition and not a very good party." He did not have to spell out the implicit comparsion with the defunct Federalists and Whigs, both of which became vestigial during the last century, once their base of support had narrowed to the Northeast.This prospect should concentrate Democratic minds and energies wonderfully in preparing for the 1982 races.

As for the Republicans, the events at Easton revealed agreement on at least one administration idea whose time may not have come. GOP officeholders tumbled over themselves to endorse virtually every new Reagan initiative except the proposed cutbacks in Social Security. Here the administration's proposals left many people more inclined to devise some bipartisan compromise revision of the system than to man the barricades for the White House.

A large majority of Americans oppose Social Security cuts. Should the administration persist, it would not be the first time that a party, fresh from electoral and legislative triumph, handed its rivals a unique opportunity for redemption.

Senate Democrats showed on Wednesday that they recognize this opportunity. After failing by only a single vote to put the Senate on record in unalterable opposition to the Reagan Social Security proposals, the Democrats forced their Republican colleagues to devise a similar resolution, which passed unanimously. It did not criticize the Reagan plan, but it put the Senate on record opposing any scheme that would "precipitously and unfairly penalize early retirees" or cut Social Security benefits more than is essential for the system's financial health.

The tactical skill displayed by Senate Democrats suggests that, on this issue at least, the party can still rally its dispirited and divided forces to fight on truly national -- and not regional -- ground. A few more such performances, and Democratic morale could again stir. Only then can one begin to determine whether the deeper Democratic ailment is terminal or temporary.