IT was a day as crisp and delightful as a freshly picked McIntosh apple. Paul Tsongas, a Democratic candidate for president, stood at the edge of a grassy field under a bright blue sky at Newport High School, where he was answering questions from several hundred enthusiastic students.What would Mr. Tsongas do about gun control? they asked. What about abortion? Missing prisoners of war in Vietnam? The federal budget deficit? Lowering the drinking age? Affirmative action? Gay rights? Where did Tsongas rank in his high school class of 700? Tsongas, a former United States senator, quietly and deftly handled them all, including the acknowledgment that, as best he could recall, he ranked between 18th and 20th in his graduating class at Lowell (Mass.) High School. That night Tsongas was undoubtedly the prime topic of conversation around dozens of dinner tables in Newport, a small community known for its woolen mills and small arms factory. For six months, until Friday, Tsongas could describe himself accurately as "the world's supply of Democratic candidates for president of the United States." Tsongas's aides say that his early start gave him important advantages over the two newest Democratic entries, Sen. Tom Harkin of Iowa and Gov. L. Douglas Wilder of Virginia. Tsongas has been particularly active building an organization here in New Hampshire, with its first-in-the-nation presidential primary next February. Susan Prolman, New Hampshire director for the Tsongas campaign, says that in the next five weeks hundreds of Tsongas volunteers, many from next-door Massachusetts, will complete a door-to-door canvass of 59,000 Democratic households - a major portion of the state's 125,000 active Democratic voters. The canvass is producing scores of new volunteers and valuable voter data for Tsongas's campaign computer. No other candidate has even opened an office here. Tsongas's early start, however, has failed to give him the kind of commanding lead in New Hampshire enjoyed four years earlier by another Massachusetts politician of Hellenic background, former Gov. Michael Dukakis, who later went on to win the party's nomination. Several Democratic insiders here expect Tsongas to face a rough road. Says one: "I don't think Tsongas has been able to make inroads [even though] he's had the field all to himself. He was clearly up here and running since March. After Michael Dukakis had been here for that period of time, he had locked up a sizable portion of the vote." Yet Tsongas, despite his low-key style, his pedagogic emphasis on issues and his self-confessed lack of charisma, has surprised the experts before. One Democrat notes: "Paul has been a very deceptive campaigner over the years. When he ran for Congress, he wasn't given much of a chance, and everyone woke up one morning and said, 'Gee, he won! The same thing happened in Tsongas's come-from-behind victory over Republican Sen. Edward Brooke in 1978. Meanwhile, in the traditional style of New Hampshire presidential politics, Tsongas courts voters one by one, as he did the other day in nearby Claremont on the Vermont border. After a walking tour of the community and visits to local factories, he sat down at a restaurant on Tremont Square for a Dutch-treat luncheon with a dozen or so local Democratic activists. Over sandwiches and soft drinks, Tsongas laid out the kind of economic message that he says is central to Democratic hopes in 1992. "I really t hink the country is headed for an economic fall," he says. "I don't want to overstate that, but ... this country is not competing. And the end result ... is going to be a rapid decline in the American standard of living. It's that simple." Earlier, Tsongas observed in an interview that Washington politicians who believe that the 1992 election is going to be simply a rerun of 1988 "are really not on the ground." Collapsing banks, failing companies, lost jobs, crumbling schools are sending shock waves through the country. It is striking communities like Claremont, where the high school, long the pride of the town, lost its accreditation because the community was unable to afford the funds to make required improvements. Tsongas rides into these communities, as he puts it, like "an economic Paul Revere," warning of danger. Democrats must be more than mere Bush-bashers, Tsongas cautions. He says that Democrats have lost elections in recent years because the American public no longer trusts the party to run the nation's economy. Democrats are traditionally for civil rights, women's rights, and the environment, Tsongas says - adding that he has a strong record in all these areas. But that liberal record has been marred by economic malfeasance on the part of a number of Democratic politicians. "The definition of liberalism, as I see it, is the expansion of the economic pie," Tsongas says. "If Democrats don't understand how to create jobs, and the pie shrinks ... there is nothing liberal about that." To bring about economic growth, Tsongas would cut capital gains taxes in ways that would encourage long-term investment. He would also encourage targeted federal spending for basic research into process technologies. These steps would encourage "massive reinvestment in our industrial infrastructure." Only in that way, he says, can America fend off competition from Japan, Germany, France, and other fast-moving nations and prevent the foreign takeover of much of this nation's manufacturing base.