The skies exploded over Madison Square Garden Monday night, a few hours after the delegates voted to be bound on the first ballot, and thus to nominate Jimmy Carter. Seventh Avenue was Wagnerian -- the lightning and thunder simultaneous, rain whipping down Manhattan canyons onto Carterites, Kennedyites and the merely curious. A television crew filmed it, bravely perched on a bank's roof. Well they might, for at that hour the show inside the Garden was far murkier, far less interesting -- except to the political fancy.

Many of those, like me, were wondering how it had come to be that an incumbent president had beaten a challenger once thought to be invincible, beaten him in state after state and at last on the roaring convention floor -- and left us curiously unsatisfied, puzzled by the evident disparity between the power and control exhibited by his forces on the convention floor and his political weakness in the country, as reported in the polls.

Equally curious was the progress of the "binding rule" itself. What had begun as an instrument of reform, aimed at giving to primary and caucus voters the power once exercised by party leaders and convention delegates, had ended as an instrument of dominion in the hands of the incumbent's campaign organization. Its defenders were perfectly right in calling it the product of a decade's revision of party rules, agreed to by representatives of both camps long before the campaign started. They were right in reminding the delegates that they were in New York because voters had chosen them to nominate Carter or Kennedy, not to be Edmund Burkes. And to the extent that might makes right in political matters, they were right because they had the votes. Not all the eloquence of Edward Bennett Williams and Hugh Carey could change that.

Williams and Carey, like many people in Washington during the past few weeks, thought a third choice should be possible. Perhaps Muskie or Jackson, or Carey or Moynihan. One problem was that none of these men had campaigned for a single vote during the long primary season. Several polls indicated that a majority of the country's voting Democrats wanted a third choice. Theoretically, delegates to a national convention should be sensitive to such polls. But that was theory, and the reality was that the delegates to the New York convention were emotionally, and -- so the Carter people reminded them -- morally bound to support their primary and caucus candidates. They were, in effect, sealed off from popular discontent, and from the speculations of political cognoscenti, by the quite legitimate political process that had selected them. (Whether that process is wise as well as legitimate is another matter.)

No doubt a further problem for Muskie, apparently the most favored "outside" contender, was that he had so recently signed on as Carter's secretary of state. To abandon that post, and implicitly to renounce the president's political leadership, would have required drastic provocation, particularly since the commitment of the president's delegates and the skill of his campaign organization would have made it a long shot indeed. Further, I believe, it would have required vigorous, public encouragement by leading Democratic senators, congressmen and governors; and it would have required -- two or three weeks ago -- Sen. Kennedy's release of his delegates, urging them to support Muskie. Neither of these events occurred. Neither did an authorized Muskie candidacy.

In the wake of this convention, thinking Democrats will be (or ought to be) concerned with two futures: the next 10 weeks, and the next four years. If the polls are accurate, the party faces a moment of maximum danger in November. Many, perhaps most, Democratic members of Congress and governors will be following nature's first law -- self-preservation -- and putting distance between themselves and the president in direct proportion to his lag behind Reagan. Sen. Kennedy should try to help; although his "four more years" speech of last week makes it difficult to perceive him as a Carter enthusiast, he can scarcely be more fond of Reagan's economic policies than he is of Carter's, and sitting this one out will cause many people to nurse grudges in future years. Gene McCarthy can bear witness to that.

To the extent there is much pro-Carter energy in the hearts of Democratic politicians today, it seems produced almost entirely by opposition to Reagan and the politics he represents. This will probably make the election close, but more will be needed if the Democrats are to win it. The Carter campaign organization, and the talents of Robert Strauss and Hamilton Jordan, are extraordinary assets. Still more is needed: a Carter who addresses the nation's needs with force and clarity; who allies himself with his party's best traditions while refusing to promise what cannot be sensibly delivered; who fixes his attention, and ours, on larger matters of public consequence, and not on the processes of his own thought, career and associations. Democrats have in Reagan someone to run from, but they will not run in sufficient numbers to Carter unless he provides positive reasons to do so. Much in his record should attract them. It is his grasp that needs convincing demonstration in the weeks ahead.

Whatever the outcome in November, Democrats must surely examine the delegate selection process that produced such a mighty clash in New York. Is it wise to bind convention delegates to the degree that they cannot behave as responsive politicians? Is a system adequate that includes so few elected officials in the delegations? Is it sensible to rely so heavily on primaries, and so little on forums in which the essential trades and compromises of politics can occur? One almost chokes before suggesting another reform commission, but in some place and manner these issues must be dealt with.

There are, as always, promising possibilities in the wings for 1984; Vice President Mondale, Sens. Kennedy, Moynihan and Hart, Gov. Babbitt of Arizona, to start the mentioning business. But before one of them -- or another strong-willed, stout campaigner -- is nominated that year, he or she will wish for a process that has brought all the elements of the party into play and that is responsive, at convention time, to the political currents of the day. That is essential to winning the presidency.