Amidst cheers of delegates, Sen. Edward Kennedy's masterful address to the Democratic National Convention reached back more than 14 years to an obscure one-liner in a speech by Ronald Reagan to pursue a strategy that generates maximum optimism now, but threatens maximum peril later for President Carter's reelection.

Kennedy aroused his first great cheer in the convention's emotional peak by quoting the Republican presidential nominee as saying: "Unemployment insurance is a prepaid vacation for freeloaders." The delegates roared when Kennedy added: "And that nominee is no friend of labor."

Students of Reagan could not remember that as one of his stock one-liners. In fact, Kennedy's researchers found it in a speech to a Fresno, Calif., Republican dinner on April 13, 1966, nearly two months before his nominaton for governor of California. There is no record the statement was repeated, and it does not represent Reagan's policy.

The Fresno discovery by Kennedy's researchers began a list of Reagan quotations read by the senator, not so hoary as the unemployment insurance statement but almost equally obscure and startling. Although Kennedy's speech read Carter's economic policy out of the party platform, its most pleasurable parts for the delegates were six paragraphs of assault on Reagan. That delighted the Carter camp; it fit their own tactics perfectly.

Roughing up Reagan is the beginning, middle and end of Carter's strategy -- a situation perilous for the president if Reagan proves as impervious this time as he has to attacks in past campaigns. Since Carter has no economic plan with mass appeal, the only alternative to help from some unpredictable foreign crisis is remorseless attack on Reagan. Thus Carter's acceptance speech hammered away at his opponent in an unprecedented fashion for an incumbent president.

Buttressing this strategy are Carter pollster Pat Caddell's latest surveys. While indicating no significant gain in Reagan-Carter trial heats, Caddell's polls show "surprising" public preception of Reagan as a right-wing extremist and rate him significantly lower than Carter when voters are asked of both: "does he care about me?"

This led to near-euphoria in the Carter command trailer among presidential operatives who view Reagan as a helpless target burdened by a lifetime of incessant talking. But less than 100 feet away on the floor of Madison Square Garden, delegates were not so optimistic.

"I'm afraid these fellows think of Reagan as another Goldwater," one important Midwestern Carter supporter told us. "I think they're badly mistaken." The pessimism over November prevading the delegates stems from doubts about the assault on Reagan as a viable substitute for a driving new idea.

The pitfalls became evident Thursday night when Carter flawed his own acceptance speech by obsessive return to attacks on Reagan. He did not approach the stridency of Vice President Mondale, who called the Republican nominee simply "Reagan" -- no ronald, no mister, no govenor, in the convention speech. "Reagan has a tendency to believe he can shoot his way to peace," Mondale told a news conference. Addressing the Democratic National Committee Friday, Carter asked rhetorically: "Will my son die in war?"

The basic theme of assaulting Reagan as a warmonger runs the constant risk of alienating the voter with language such as Mondale's. But in addition to stridency lies the danger of irrelevancy.

The Fresno quotes found by Kennedy's diligent researchers were overlooked even by Gerald Ford's tough-anti-Reagan campaign in 1976. Reagan's stance against unemployment insurance, plus the quote associating the New Deal with fascism (cited by Mondale as well as Kennedy) and the call for voluntary Social Security, share a common defect; they are not Reagan's established policy, now or previously. He can be accused of silly exaggeration and occasional fatuousness. But this is not the stuff of a broad-based campaign.

Herein lies the inherent danger of a treasure hunt into musty clippings and tape recordings to find skeletons in Reagons rhetorical closet: the prizes seldom have much to do with current issues.

As Carter declared in his acceptance speech, there are fundamental differences between him and Reagan on foreign policy, defense, taxation, government regulation and the environment. But because they think it will work and because their record proves they like to campaign that way, Jimmy Carter and his men -- perhaps to their own peril -- will concentrate on painting Ronald Reagan as a missile-brandishing ignoramus.