It's no accident that, as a Soviet-American summit nears, each side nervously wonders whether the other is ready for the new relationship it has in mind. Changing such a central feature of national policy necessarily upsets the whole political status quo. It's a huge strain, and in both capitals it's showing.

Just a month ago, Mikhail Gorbachev seemed to retreat from a summit. Just as suddenly he got back on track. There could be no clearer sign of a significant foreign policy challenge to Gorbachev. A few weeks later, his prote'ge', Boris Yeltsin, lost his post. Whether the Yeltsin affair had a specific foreign policy angle we don't know. History and logic suggest a connection.

So it is necessary to ask not just what Gorbachev's foreign policy is but whether he has the political strength to sustain it. Reagan may be relieved that the Yeltsin affair has shrunk Gorbachev's superman aura, but he could not want the Soviet leader to lose his status as a viable Soviet interlocutor.

The most telling immediate answer may emerge over Afghanistan. Not only is Moscow under pressure from Washington to match arms control progress with regional restraint. For some time the Kremlin has been cultivating specu-lation that it is prepared to liquidateits costly Afghan military investment. A decision that large and risky would be hard evidence of a Kremlin leader's clout.

Meanwhile, a mirroring question is being asked in Moscow about Ronald Reagan. Will the challenge to him from his right hobble his reach for agreements? The withdrawing or the nonmaterializing -- whatever it was -- of an invitation to Gorbachev to address Congress propelled this question from quiet murmur to public rejoinder in Moscow the other day. A government spokesman cited ''forces opposed to any change for the better in U.S.-Soviet relations,'' anxiously describing them as influential but not predominant.

A Soviet spokesman would hardly say, on the eve of a summit, that ''anti-Soviet forces'' were predominant. That would mean Gorbachev had made a huge mistake in cranking up for a major diplomatic encounter with a figure, Reagan, whom his predecessors had anathematized. But has he?

There is a hidden premise here. It is to the benefit of each leader that the other be strong and successful, to a point: not so strong as to be impervious to dealing and compromise but strong enough to make and defend a deal against the pressures of those who are rivals or who think the project is unwise. In this familiar way Reagan and Gorbachev have acquired an interest in having each other's political fortunes boosted.

Reagan is on warning here. His party is full of people who suspect not only that he is woolly headed and not very smart -- they think of Reykjavik and shudder -- but that his constrained political circumstances make him painfully vulnerable to the temptations of a bad deal with Gorbachev.

Hovering over this whole scene is a common awareness of the breakdown of de'tente in the 1970s -- and an abiding disagreement over the cause of the breakdown. Americans usually attribute it to a Kremlin decision to exploit America's Vietnam distraction by building up arms, encouraging Third World revolutions and otherwise projecting Soviet power.

At least until recently, anyway, Soviets hesitated to concede that their appetite for expansion and their arbitrariness on human rights had something to do with it. They found it easier to attribute the breakdown to weakness in the White House: first it was Richard Nixon, who could not deliver trade, then it was Jimmy Carter, who couldn't deliver arms control; neither could contain human rights interventions.

It seemed Gorbachev had decided to bet on a strong conservative in the White House. But now he can see that Reagan has lost both houses of Congress, remains under Iran-contra siege and, as an election year nears, is having to turn to his opposition to carry a treaty under heavy assault within his own party.

Some Americans worry that the Soviets look at a flailing Reagan and think he's a rube who can be taken. I suspect they have had enough experience with deals with the United States that are made and come undone to want one that sticks.