President Carter, campaigning last Thursday in two crucial states where he now trails, spent 12 hours escalating misrepresentations about Ronald Reagan as a mad nuclear bomber, but a questioner suddenly brought him up short, showing the limits of this central tactic of the president's campaign.

A bone-tired Carter was asked, during a live interview over Philadelphia's WCAU-TV whether it was not "kind of harsh" to suggest a vote for Reagan would be a vote for war. "That would be too harsh, yes," the president replied quietly. "That certainly would be."

Carter had suggested exactly that a week earlier. He kept insinuating it over and over last Thursday, but soon discovered that this is not the same as trashing Carl Sanders in Georgia in 1970 or even Teddy Kennedy in last spring's primaries. The president's words are being so closely scrutinized that he cannot get away with all his old tricks.

Since Carter must back away from the overt warmonger accusations, his attack ultimately boils down to policy disagreements with Reagan over the merits of SALT II. In a political climate that demands stronger defense and tougher diplomacy, that disagreement is not necessarily a political advantage for the president.

Carter's managers believe he has no alternative. It is not that he trails Reagan so badly but that he lacks both momentum and an overriding campaign theme. Unlike other Democratic presidents, he cannot fall back on bread-and-butter issues. So the Carter camp relies on the tactic it knows and practices best: assault the opponent.

That was the situation as the president began a day-long swing through Ohio and Pennsylvania, two states he won in 1976 and may have to win in 1980. In each, he trails in the polls by around 5 percentage points (about the same as his national deficit).

Guided by Pat Caddell's polls, the new wave of Carter television spots seeks to exploit public doubts about whether Reagan as president would take the nation to war. They bear Gerald Rafshoon's inimitable trademark of relentless attack.

On the eve of Carter's Ohio-Pennsylvania trip, television viewers across the country saw variations of two basic Carter commercials: a half-dozen or so California voters, interviewed on the street, worrying that their former governor would "shoot from the hip" as president; a review of past Reagan statements threatening to show or use military force. Jimmy Carter himself? He was scarcely mentioned.

Reagan's interview with the Associated Press that day coincidentally provided ammunition for the president's trip. Opening the day with a Dayton "town meeting," Carter called Reagan's reiterated opposition to SALT II "a departure" from "a national commitment to peace." By the end of the day, Carter had refined his charge. "Gov. Reagan announced that he was going to abandon the SALT II agreement and replace it with an American nuclear arms race," he said in that TV interview.

In fact, Reagan told the AP that because he believes SALT II favors the Soviets, he would immediately seek new arms negotiations and simultaneously increase arms production as a bargaining lever. That basic disagreement over arms-control strategy and policy was twisted by Carter into an accusation that Reagan does not want any arms control at all.

This is traditional and habitual for Jimmy Carter. Reagan's 1975 suggestion that a destroyer might accompany U.S. tuna boats to Ecuador if a fishing rights dispute were not resolved now becomes, in the president's words here, as follows: "He advocated sending in the American Navy."

The tactic transcends military affairs. At a Philadelphia fund-raiser, Carter quoted Reagan in the AP interview as advocating repeal of the oil windfall profits tax and abolition of the Department of Energy. In fact, Reagan said the former was not possible now and he "would look very closely" at the latter. "He [Reagan] wants to eliminate conservation measures including the 55-m.p.h. speed limit," said Carter. In fact, Reagan is not specifically on record for any such elimination, including repealing the speed limit (which is contained in the Republican platform).

This Carter style irritates more than editorial writers. In suburban Lansdowne, Pa., a woman questioner contrasted Carter's past "upbeat and positive" tone to "negative aspects . . . in political television commercials attacking your opponents" -- specifically, TV commercials condemning Rep. John Anderson for his "Jesus Christ Amendment" 15 years ago.

"I have never heard of the TV spot that you refer to about Anderson and the Jesus Christ Amendment," Carter replied coolly. He was right. It is a radio commercial, played heavily in Democratic areas. The president's answer reflected his campaign style. The campaign is intended to eat up Reagan's lead, but in fact it was arousing hostility from the Philadelphia television interviewer, the woman in Lansdowne and, very likely, a great many other voters who expected something better.