The most volatile election campaign in modern times goes into its final day today, with Republican challenger Ronald Reagan holding a 3- to 5-point lead over President Carter in two national polls but wondering -- along with everyone else -- if the latest turn in the year-old Iranian hostage story could also be the final plot reversal in the election drama.

In a brief but emotionally charged television talk from the White House last night, Carter vowed that the delicate negotiations for the release of the 52 Americans held in Tehran since last Nov. 4 "will not be affected" by the election calendar. He said he was keeping Reagan and independent candidate John B. Anderson informed on the talks while they continued their campaigns. And he told Iranian authorities Americans would remain united on the issue no matter who wins tomorrow's election.

Reagan and his aides heard the Carter statement on TV en route from Dayton to Cincinnati, conferred briefly and then decided to make no comment.

As the White House announced tentative plans for Carter to resume early today the cross-country campaigning he broke off in Chicago yesterday, no one in the Carter camp would venture a serious guess on how the developments might affect his uphill battle to reverse Reagan's apparent advantage in the White House race.

Reagan campaign manager William Casey said, "The campaign is over. We're just playing it out." But other Republicans worried privately that the climax of the hostage drama might play into Carter's hands just as much as its onset did a year ago.

Then, trailing badly in the polls behind Democratic challenger Edward M. Kennedy, Carter invoked the same sense of national unity he spoke of last night and crushed Kennedy's campaign with stunning ease.

Later in the spring, using language similar to last night's, Carter announced hopeful developments on the morning of the Wisconsin primary. The tactic gained him votes there but left a residue of cynicism that may reduce or eliminate any political benefits from yesterday's events.

Pollsters George Gallup and Louis Harris, who reported Reagan's lead in surveys completed Saturday, cautioned that such a reversal was possible again. "Never in the 45-year history of presidential election surveys has the Gallup Poll found such volatility and uncertainty," Gallup said.

The Gallup Poll taken between Thursday and 2 p.m. Saturday and released yesterday gave Reagan 47, Carter 44, independent John B. Anderson 8 and other candidates 1, based on expressed preferences and allocation of a 3 percent undecided vote.

Gallup said the reported difference was so small that it fell within the statistical margin of error and Carter might have been a fraction ahead before the announcement from Tehran of the terms for releasing the 52 Americans held there since last Nov. 4.

The Harris Poll was outside the margin of error: Reagan 45, Carter 40, Anderson 10, others 1, and 4 percent undecided. That was based on interviews Friday and Saturday, and apparently was at odds with a survey by Carter pollster Patrick Caddell those same days, which campaign sources said put the president up 1/2 to 1 1/2 points.

Those officials interpreted the Caddell findings as proof that the 5-point gain most surveys found for Reagan immediately after last Tuesday's Reagan-Carter debate had dissipated by the end of the week.

But polls taken as late as Saturday by Republicans showed Reagan holding a 5- to 7-point margin in such battleground states as Michigan, Missouri and Pennsylvania, with Ohio still a virtual deadlock.

Correspondents with Reagan in Ohio noted that the relaxed and jubilant atmosphere that prevailed Friday and Saturday in a campaign entourage plainly confident of victory had given way to a much more jittery and tense mood.

While continuing his criticism of administration economic policies, Reagan omitted personal references to Carter and told reporters the hostage situation is "too sensitive to talk about."

For weeks, Reagan and his campaign aides had predicted Carter would produce an "October surprise" to aid his campaign, suggesting that a hostage development was the most likely. Special commercials were prepared weeks ago for use if the hostages returned. They reportedly were designed to turn viewers' minds to the question of who was responsible for their long incarceration and the "shame" the incident had brought on America.

But Casey and other officials said last night the commercials would not be run. "All of us hope the hostages will get out somehow," Casey said, "but frankly I don't think it will make that much difference at this stage of the campaign."

Despite those disclaimers, the Reagan camp began trying to deal with the political fallout of the hostage development within hours after the announcement from Tehran of the terms of release.

In back-to-back appearances on CBS' "Face the Nation" and NBC's "Meet the Press," vice presidential nominee George Bush and former president Gerald Ford gave almost identical messages to the voters.

While making no overt change that Carter was manipulating the situation for his benefit, both said the president had played politics with the hostage issue earlier in the primaries, and Ford said "it seems like a coincidence that this great activity is taking place just now."

Ford said Anderson would "really resent the ability of a foreign nation, in this case an ayatollah, to affect the election of the president. . . . They really resent the manipulation whether it comes from the ayatollah . . . or the president in his very human desire to get reelected."

Bush suggested that the Iranian authorities might prefer to deal with Carter because "maybe they see Ron Reagan as a more principled man . . . who would want to keep our word of honor. . . . I don't think Ronald Reagan would be very tolerant of the U.S. being held hostage for a year."

Both Ford and Bush said the general terms offered by Tehran provide a reasonable basis for negotiation. But both condemned in advance any agreement to the phased return of hostages, saying all should be released before the United States complies with any of Iran's conditions. In addition to dominating the Sunday talk shows with their comments on the hostage issue, the Republicans may have the last word on the subject tonight. Reagan is scheduled to tape a speech in a Peoria, Ill., television studio this morning that will be run in 20- or 30-minute versions on all three networks tonight. Aides said last night it would give him an opportunity to discuss the issue if he decided there was need to do so.

Carter has already recorded a campaign closing speech for showing on ABC tonight, and since the speech is integrated into a 20-minute film by his media specialist, Gerald Rafshoon, aides said it might be more difficult to update it to include a reference to the hostage situation. Carter's talk, like the one originally drafted for Reagan, is a general speech on leadership for the future.

A Reagan official predicted, early in the day yesterday, that before the day was over, "Carter will be on television with a report to the people on the hostage crisis. He'll say the Iranians misjudged him in thinking he would accept any terms for their release that might jeopardize the national interest, just because he was on the eve of an election. He'll say his own election is not important; just the honor of America and the lives of the hostages. And he'll use the word crisis six times."

Except for the reference to "crisis," Carter covered most of the predicted points.

But some Carter campaign officials said that even if the events gave Carter a unique platform from which to dramatize his claim to being an effective, experienced national leader, it was not without its costs. They referred to the diminished attention to what they call the "contradictions" in Reagan's positions -- the subject of a new ad the campaign just started airing -- and to the fact that Carter was unable to make a day of campaign appearances in vital Great Lakes battleground states.