America's longest campaign has ended and the nation decides today, at last, who will be its president for the next four years.

For Jimmy Carter, the Democratic incumbent, the election has come down to a far-from-certain hope that last-minute conversions from the ranks of the undecided will enable him to remain in the Oval Office for another term.

For Ronald Reagan, the Republican nominee, the election has come down to the hope that no last-minute decisions in Iran -- will cause his candidacy to lose the closing momentum that has seemed to be clearly his.

For John B. Anderson, the independent challenger, the election has come down to an ending seemingly without hope of victory -- but with the perhaps ennobling conclusion that his candidacy has had a measurable effect on the two-party system, the measure of which may be the unseating of the incumbent.

And for the nation, there is now the prospect of Republican gains in a number of Senate races, perhaps even enough to bring the Grand Old Party close to capturing control of the Senate, a status they have not enjoyed since 1954.

In the final hours, the race for the presidency had come down to a struggle to win the hearts and minds -- and mainly, the votes -- of those people who had been telling pollsters for months that they were undecided. And final tracking surveys by poll takers for Carter and Reagan showed that as of Sunday night, Reagan seemed to be opening up a lead.

The president's advisers had been counting on a shift of this year's volatile electorate during the campaign's final weekend, even if Reagan were ahead. "Give the public that last weekend to contemplate the prospect of a Reagan presidency and they will turn away from him," Hamilton Jordan had said. But apparently the public did not respond quite that way.

According to an informed source, polls taken by Carter pollster Patrick Caddell showed that the president had been gaining on Saturday night, but by Sunday the movement was swinging perceptibly toward Reagan. The figures of Reagan pollster Richard Wirthlin were said to show the Republican nominee in the lead and moving comfortably away.

But even with these figures, there is a crucial caveat: The statistics could not take into account last-minute movement that might have resulted from Sunday's dramatic and optimistic events concerning the fate of the Americans who have been held hostage in Tehran for precisely one year today.

Although the choice of timing had rested with the leaders in Tehran and not in Washington, Carter's strategists have been hoping that the president would reap an 11th-hour benefit from the apparent prospect that the hostages will soon be freed.

Carter, taking care to stress statesmanship and not candidacy, spoke briefly over nationwide television Sunday evening to say that Tehran's new statement of conditions offered a "positive basis" for an early release of the hostages. But he cautioned that he would make no decisions with an eye toward the U.S. election. And as the president returned to the campaign trail yesterday for one last run, neither he nor Reagan sought to use the hostage issue for political gain.

It was in this unique setting that most Americans ushered in the last hours of a campaign that had often been anything but presidential. It had been a campaign that had lasted longer than any other in modern American history, well over a year. And it had been a campaign in which all those running found themselves striving repeatedly to prove themselves "presidential" -- and that included the incumbent, a fact that said much about the nature of Carter's difficult struggle to win reelection.

It is also crucial to understanding the nature of this campaign to recognize that the undecided voters of the fall of 1980 were not actually voters who had made no decision about the campaign. They were, perhaps more than ever before, people who were well informed (not at all apathetic, as perhaps was the case with the "undecided" poll respondents in years past), and they were people who had already made one crucial early decision: They had looked at the incumbent and decided that they did not really want to vote for him, if they could only find someone more acceptable.

So they looked for a while toward Anderson, and then they looked for a time toward Reagan, and they found flaws in each. And so they returned once again to the category of the undecideds -- and it was from this pool that the strategists agreed that the election of 1980 would eventually be won.

The volatility of America's electorate in its search for a candidate of its choice produced some often startling incongruities. The nation's Jewish voters, for instance, in their longstanding discontent with Carter, found themselves flocking toward the candidacy of Anderson -- who in three straight Congresses spanning a six-year period had introduced an amendment to the Constitution that said:

"This Nation devoutly recognizes the authority and law of Jesus Christ; Savior and Ruler of nations, through whom are bestowed the blessings of Almighty God."

Many Jewish voters have since returned to the Carter ranks, but a significant number of Jews are still expected to break their traditionally Democratic voting habits and cast their ballot for Anderson (also a born-again Christian, but not a Southern one) in numbers that may be large enough to cost Carter a victory in some key states.

So, too, there was the incongruous specter of the former governor of California criticizing overregulation of automobile emission standards -- buttressing his claim with the contention that trees cause more pollution than automobiles, and so does Mount St. Helens -- and then flying home to his own greater Los Angeles only to have his own plane be diverted from landing due to smog pollution.

But, perhaps most significant, was the ongoing image problem of the non-presidential president. And it was here that Carter seemed to have done himself little good in his long-awaited debate just a week ago with Reagan. More than anything else, that debate may have swung the campaign-ending momentum Reagan's way.

By the very fact that the 69-year-old Reagan, skilled in the performing arts, was able to stand on his feet and respond to questions with the appearance of substance and confidence, he succeeded in greatly reducing the concerns of many viewers that it would be a risk to have him as president. Carter, meanwhile, made precisely the sort of debate gaffe that his strategists had long hoped Reagan would make.For 89 minutes, the president had debated with point-by-point skill, keeping Reagan on the defensive. But in one moment, as though overcome by his desire to appear folksy and human while attacking Reagan's militaristic inclinations, Carter, explaining that halting the proliferation of nuclear weapons was the No. 1 issue of our time, said he knew this because his daughter, Amy, told him so.

The presidential debate of 1980 will always be remembered for this; it became the joke of the campaign. But back in their headquarters, the Carter strategists found it decidedly unfunny. "It trivialized the main point he was trying to make," one senior Carter adviser said sadly. He could have added, as well, that it also diminished the stature of the president.

On Sunday, Carter continued to suffer from this moment of nonsubstantive hyperbole, even as he was was interrupting a football telecast to make his truly presidential statement in response to the declaration from the leaders in Iran. Shortly before, the nation of football watchers had heard former Dallas Cowboy's star Roger Staubach, a television broadcaster who happens to be a staunch Reagan supporter, offer this explanation for the year-long problem of one of the terms on the field:

"In fact, I talked to my daughter Amy this morning about it and she said the No. 1 problem was the bomb."

The presidential debate of 1980 had made its mark on the political conscience of the nation.