Only a half-dozen or so U.S. senators are among the delegates assembling here for the Democratic convention. And only one-third of the Senate's 100 seats are up for election. So the World's Greatest Deliberative Body is not in a position to have much direct influence on this week's Democratic Party deliberations.

But the Senate's voice in treaty-making gives it a particular concern for this country's foreign policy. So it is that in the rarefied air of the Upper Chamber you encounter an interesting argument, in foreign policy terms, against the "open convention" campaign -- the movement to dump Jimmy Carter.

It has to do with appearances, with how the United States comes across as a world power, capable of managing its internal affairs in an orderly and comprehensible way, which has almost everything to do with its ability to manage international affairs.

And the appearance that troubles a significant number of Democratic senators, judging from my soundings, is that of a nation regularly destroying its presidents.

As the demolition squads maneuver against the Carter presidency, consider the immediate historical context: First, the rejection of Gerald Ford after only two years in office. And before that, the removal of Richard Nixon -- in disgrace, true enough, but even some of our most enlightened foreign firends still don't understand that. And before that, the forced withdrawal of Lyndon Johnson (who had replaced a president whose own term had been brutally cut short by assassination.)

Five presidents in 20 years.

"It's awful, what this is doing to us overseas," says Sen. Abe Ribicoff (D-Conn.), who will make the opening speech on behalf of the Carter forces in the "open convention" debate tonight. Ribicoff's voluntary retirement at the end of this term may make him a poor witness on the question of his party's need for a winning candidate. But his electoral detachment may lend a certain clarity to his thinking.

With more time for travel and reflection, and no immediate personal stake in who is at the top of the ticket, Ribicoff makes at least an interesting witness on the "open convention" question. He thinks it's wrong to "phony up an issue and destroy a president -- I don't think this country can afford to be seen destroying presidents."

Now you can say that it's nobody's business but ours, and strictly speaking, that's so. Or you can argue that appearances abroad are less important than the free play of hard-nosed practical politics at home, and that's so.

But the point made by Democratic senators I'm talking about is that repudiating Carter doesn't make sense on either cournt.

Delaware's Joseph Biden, for example, agrees entirely with the Ribicoff argument on the unseemliness of destroying presidencies. But he doesn't like to make that argument, he says, because it implies a weakness in what he considers to be a more persuasive argument in practical political terms. Biden thinks Carter, for all his liabilities, would be the strongest available candidate.

And he is not alone. A week ago, a mixed bag of about 15 Democratic senators gathered with Majority Leader Robert Byrd to kick around the choices confronting the convention. There was some musing about a senatorial delegation calling upon the president, perhaps urging his withdrawal. The talk turned to alternate candidates.

"Not a single person in the room thought the party would do better with anybody other than Carter," one participant reports. "One after another, trial balloons were floated -- and shot down."

A few days later, Biden and Sen. Walter Huddleston (D-Ky.) fell to talking about the "media erosion" of Carter's position in the Senate (as Biden puts it) and agreed to take an hour or two to sample sentatorial sentiments. When they checked back, they had reached a total of 23 Senate Democrats. Among them, reportedly: Agleton, Glenn, Nunn, Chiles and Bentsen. A few say no harm and perhaps even a tactical advantage for Carter in an "open convention," according to Biden. "But not one say anything to be gained in dumping Carter."

The consensus, Biden claims, was that Sen. Edward Kennedy wouldn't be nominated, and would be a weaker candidate than Carter in any case. It was generally agreed that none of the alternatives, who would be starting from scratch with no money or organization, would be any stronger than Carter, given the damage that would be done to the party in the process of rejecting the president.

That's the real message from the congressional Upper Chamber to the convention floor. It may be one thing to destroy a presidency, and never mind the spectacle, if there's some practical political gain. But there's no sense, as Ribicoff puts it, "in having all this blood in the water for nothing."