When does Jimmy Carter's sinking stock touch bottom and begin to bounce back? That time comes, I think, when his own standing becomes visibly entwined with the country's welfare.

A case in point is the Mideast summit, sheduled for next month, which will almost surely be a plus for the president. But it is hard to see how he can develop a similar kind of event for the truly critical question inflation.

Carter's fall in the polls in different from anything the country has recently experienced. When Lyndon Johnson began to go down, everybody knew it was because of Vietnam. When Richard Nixon tumbled, Watergate was not exactly a secret. The role of the pardon in setting Jerry Ford on the slippery slope was equally obvious.

But Carter's drop has been unconnected with seriously adverse events. Peace and prosperity have marked his presidency. He has been measured against expectations, far more than actual developments. He has lacked the bracing stimulus of reality, the sense that what happens to him matters to all of us in ways that can cause us to pay or bleed.

Events in the Middle East have now visibly cracked through the cottonwool of unreality. President Anwar Sadat's dramatic trip to Jerusalem brought the area into the American living room. The withering of hope and the mushrooming of new dangers is almost as familiar. Who does not know of the possibility that Lebanon could explode? Of the bloody arguments raging among Syrians, Iraqis and Palestinians? Of the stridently hostile tone that characterizes recent relations between Sadat and Israeli Prime Minister Menachem Begin?

Those perceived perils make an ideal backdrop for presidential leadership. Carter has economic leverage to bring to bear on Egypt, and the control over arms supplies that makes the clout with Israel. American public opinion will clearly support any measure that arrests the slide toward a new round of war.

So at the Camp David summit, Carter can do something meaningful. He can - and I think the odds are fair that he will - foster a resumption of negotiations that changes the bleak perspective. If so, his stock in the country will rise dramatically.

Even if he fails, moreover, the blame is apt to fall on Sadat or (more likely) Begin. The president will get credit for trying. He might even enjoy the kind of paradoxical rise in the polls that came to John Kennedy after the Bay of Pigs fiasco.

But the bottoming out that seems likely in foreign policy does not find a domestic counterpart. Inflation is not something foisted upon good guys by bad guys. Much as we might like to blame the oil sheiks, furthermore, the principal villain is not in some foreign land.

On the contrary, a multitude of small, almost anonymous forces push up prices and wages. The do not jump; they creep. Most efforts to tamp them down are more harmful politically than helpful as anti-inflation medicine.

Business confidence is now so shaky, and the myth of its importance so well established, that the administration does not even dare say a nasty word about price increases. Hitting labor is less unpopular, but more damaging to the president's own supporters. Thus Carter has had to call off the shots justly taken at the unions by Barry Bosworth, the director of the Council on Wages and Prices.

Worse still, from the presidential point of view there is no evident point of confrontation with Congress. If the administration, taking a conservative stance, tries to hold the line on health outlays one day, Congress, also taking a conservative stance, breaches the limit of defense spending the next. If the administration talks about vetoing a tax bill that does big favors for the rich, Congress dishes out even bigger favors to the homeowners who make up such a big part of the electorate that a veto becomes virtually unthinkable.

Perhaps some self-evident crisis will supervene. The flight from the dollar could reach panic proportions. Inflation could zoom. In that case there would be an opening for an action by the president that would put his own success or failure in visible relation with the well-being of the country.

But such dramatic doings are not now on the horizon. In their absence, Carter can do little to turn his fortunes around except luck out.