THAT WAS a very rare interview -- one on one -- that Konstantin Chernenko gave Washington Post correspondent Dusko Doder. It was the first in Mr. Chernenko's eight months as Soviet president. And precisely because of its uncommon nature, the question is quick to arise: why now? The answer has several parts. In the background may lie Soviet concern over the extra strategic and economic burdens of life after d,etente. In the middle ground there is the condition of flux that led Soviet Foreign Minister Andrei Gromyko last month to test the possibilities of intervention in the American political campaign. In the foreground -- the here and now -- is the evident tightening of the race and a resulting Soviet hope to get the two candidates to bid each other up a bit in the debate on Sunday for the title of most likely to deal peaceably and effectively with Moscow in 1985.

But will it work out that way? The 73-year-old Chernenko and his advisers surely know that the "practical steps" he now asks of the United States -- demilitarizing space, a nuclear freeze, a pledge of no first nuclear use, and ending all nuclear tests -- require from Ronald Reagan extremely difficult policy reversals. In that sense the Chernenko agenda implicitly gives a broad opening to Walter Mondale, but it can't be considered a very attractive one. Mr. Mondale can hardly be eager to become sponsor of any part of the offered Soviet negotiating position.

What then can either candidate profitably say in response? President Reagan will probably argue that it is his tough line that has induced the Russians to stop huffing and puffing and to knock again on the Washington door, to propose an intermediate agenda and to hint that only parts of it need to be taken up in order to restart the big missile talks that Moscow quit last year. Mr. Chernenko did seem to be easing the previous Soviet insistence that, for these talks to resume, the United States must first have rolled back its new European missile deployments.

Mr. Mondale will probably point out that all Mr. Reagan has really done in Soviet-American relations is finally to stop making them worse. His main task will be to persuade voters that he has the firmness as well as the flexibility to break the arms control stalemate on terms satisfactory to them.

The Chernenko approach, which expands on Soviet formulations made earlier this year, is face- saving and in some respects practical: to tackle other arms control issues now by way of sliding back later into talks about the missiles that matter most. Of these other issues, however, all but one are really out of the question. That one is the matter of underground nuclear tests. The two sides could conceivably move to complete negotiations on banning underground tests, or the United States could, on the basis of some new talks and understandings, move to ratify the two already-negotiated treaties (1974, 1976) limiting the size of tests pending a full ban. This is an old debate. The weapons testers have their reasons for opposing ratification of these treaties, and we will probably be hearing much about them in the days ahead. But so far as we have been able to judge, none of these reasons for refusing to go ahead on the underground testing limitations stands up well.