Why is Jimmy Carter having so much trouble with the Senate? Wasn't it a staple of his campaign argument that, say what you might about the candidate himself, only a Democratic President working with the Democratic Congress could end the political stalemate that had characterized Gerald Ford's Washington? What happened, then? And what are the prospects for smoother sailing for the Carter program in the Senate - currently the scene of the mauling of his energy proposals and in time to be the setting of crucial debates on the Panama treaties and a second-stage SALT accord?

I judge the prospects to be pretty terrible, at least for anything even dimly resembling an easy, one-for-all-and-all-for-one party relationship between the Senate and the White House. For a variety of reasons this particular Senate and this particular President are just not destined to get along that way. These reasons are somewhat different from the explanations offered up last week as the Senate sank into its low-grade-fever filibuster - ie., that the Senate was lacking in internal party discipline and an ability to rise above parochial interests; that the President didn't understand Capitol Hill; that he failed to consult in advance and build a bipartisan consensus for what he wanted; that he was too stiff-necked and wishy-washy (both); that he refused to be selective in his legislative demands, or even to establish priorities, instead choking the Senate with complex and politically difficult proposals.

I'm not saying all this is untrue. Probably, in some degree, it's all true, even the seemingly contradictory parts. But these are merely the earth tremors, the jolts and shakes that are sending some of the china crashing to the floor. The geologic fault underlying it all, that great shifting of weight and tension within the Senate and between the Senate and the presidency, is something else again. And as Jimmy Carter's luck would have it, both the ways in which the Senate has changed and the ways in which it has not bode ill for any hope of easygoing political relations.

What has remained fixed about the Senate - its size and, by and large, its folkways and its rules - makes the institution more susceptible than the House to lobbyists or small bands of legislators who opposed what seems to be the majority's will. "You only need 10 or 15 guys with you," as a labor lobbyist puts it, "to tear hell out of the Senate." Less tangible, but as important, is the fact that the Senate's peculiar spirit - its haughtiness, sensitivity to slight and rather grand sense of self - is still intact. It has outlived the departed old-style senatorial figures to be reincarnated in the new.

I was struck by how much of the courtly, even preposterous, mannerism the old boys made famous lingers on, as I watched the rather aimless filibuster scene from the gallery the other evening. Even in their stocking feet, senators in the chamber are always "on," always looking as if they were aware of their importance, always engaged among themselves in a kind of forced, touchy-feely bonhomie - a back pat here, a handclasp there, a playful minishove, an earnest clutch of the other fellow's lapel.

The Club: It lives. Senators of every style, age and political persuasion love to tell you about how various forces and individuals outside the Senate made a terrible mistake by offending that august body or inconveniencing it in some fashion. And this seemingly immutable fact of Senate life, this preoccupation with the entitlements and independence and importance of the institutions (never mind that it doesn't always look so hot to the public), has been immeasurably strengthened by something that has changed. I am referring to the simple, central fact that Jimmy Carter has been elected President, that he is probably good for eight years in the job and that he has brought with him a young, politically live Vice President.

That fact has had a profound effect on the Senate, which for the past decade has been a staging ground for presidential candidacies, candidacies incidentally that created their own pressures toward party unity and reconciliation. No more. The chamber is strewn with dashed hopefuls. I counted seven on a roll-call vote the other day. Ambition now lies elsewhere. There is a kind of burrowing into the legislative mode. The Senate has ceased to be a stepping-stone. It is home.

For this strengthened institutional identificaton, West Virginia's dour, businesslike Robert Byrd is a fitting spokesman and leader. His loyalty is to the place, not to any collection of programs and policies it might nudge into law. Byrd, who has had some very scratchy conflicts with Carter, is by no calculation an Administration man. He rose to the top, beating out such party luminaries as Ted Kennedy and Hubert Humphrey, by mastering the institutional levers of power; and his leadership is in the same pattern. He does not beat on chairmen or identify himself with any ideological point of view. He tries, as he likes to say, to help the Senate "work its will."

Byrd's substantive neutrality may be just as well, considering the key change in the Senate - namely, that almost all the interesting and important conflict, now is among Democrats, not between them and the Republican minority. That altered political terrain is as responsible as anything for the fact that Carter doesn't have a ready-made party consensus within the Democratic majority. The big North-South civil-rights fight may be over, along with the rending, internal Vietnam quarrel, but on important foreign-policy issues that will engage the Senate, such as detente and arms control, it is "Scoop" Jackson Carter must pacify. And on the most important domestic issues it is Russell Long.

Long's power, in fact, is at least in part a consequence of another change in the Senate: With the advent of the new budget procedures, the old Appropriations Committee moguls have declined somewhat in power - and the taxing committee (Finance, of which Long is chairman) has correspondingly gained. The more the President decides to do through tax pressures and incentives - energy, health care, welfare and so forth - the more business he will be sending to Long and his committee. And Russell Long has made it just as clear as Byrd and Jackson have in their different ways that he will do it his own way.

You will have noticed that most of what the Senate is expressing has a counterpart, or even mirror image, at the other end of Pennsylvania Avenue. The independence, the innerness, the self-certainty, the insistence on staying aloof from burdensome party obligations and connections - these are elements of the Carter style as well as of the contemporary Democratic Senate. That is why I would guess that as more of the important action moves to the Senate this winter and spring, the relationship is not going to get any easier.