TOM HARKIN, the newest Democratic presidential candidate, says the 1992 election is going to be "fun," and it quickly becomes apparent what he means: Senator Harkin is going to poke fun at the person he says is America's best friend of the rich, "George Herbert Walker Bush.""Bush has feet of clay and I'm going to take a hammer to them," Harkin told a rally here this week as he launched his campaign in the Granite State. The senator, a fiery Iowa populist, is often compared to Harry Truman, whose tough talk and common folk appeal helped upset Republican Thomas E. Dewey in the 1948 presidential campaign. Could it happen again? Political scientist Thomas Mann says Harkin has the kind of "take no prisoners" style that could make waves in the Democratic primaries, but might not play well in the general election campaign next fall. But Harkin scoffs at those who question his outspoken liberalism and his criticism of Bush, whom he often refers to sarcastically by his full name. Harkin relishes his liberalism, and his battles for little people, including the disabled. He is a champion of labor unions, and embraces their support enthusiastically. He says it's time for Democrats to stop apologizing for their values. "If being for decent jobs and housing for people, education for our young, health care for all, rebuilding and making a new America is a liberal, I'm proud to be a liberal," he says. Although he is a former Navy jet pilot, Harkin has gained a reputation as one who would cut military spending sharply to pay for better housing, highways, and schools. At a sidewalk press conference here, he explained: "I intend to ask the American people to make a choice. If they want to continue to spend $160 billion a year of their money to defend Europe from the Soviet Union ... they can vote for Bush," he says. "But if they believe like I do that Europe is strong enough and rich enough and powerful enough to defend itself, and that we can take some of that $160 billion a year and start building better roads and bridges and mass transit and building energy systems here in this country and better education and health care for our people, then they ought to [vote for] Harkin." During his brief foray here, Harkin attempted to turn the rhetoric of Ronald Reagan and George Bush against the GOP. "It's time for us to measure the Republicans by their own yardstick," he says. "Remember Ronald Reagan? He came to the American people 11 years ago and he asked, 'Are you better off than you were four years ago? Harkin quips: "Our turn!" Needling the Republicans, he says: "You've had four years, George Herbert Walker Bush and J. Danforth Quayle. Are your roads any better? Are your streets safer at night? Are your schools better? You got better health care in America? How about jobs, you got jobs that put more money in your pocket?" "No!" shouts a Manchester, N.H., crowd of 200. Fortuitously for Harkin, the New Hampshire economy has soured. When Bush was seeking the presidential nomination here in 1988, the unemployment rate was just 2.6 percent, the lowest in the nation. Today it is 7.3 percent, above the national average. Back then just 15,622 people were unemployed here. Today it has rocketed to 46,699. Unemployment offices are crowded. At an employment security office in downtown Manchester, people cluster around a bulletin board when the latest job offerings are posted. Many here are losing their homes, unable to make payments. Some 50,000 jobs have been wiped out here since Bush was elected. Dr. Mann, a scholar at the Brookings Institution, says: "People are hurting and under those conditions, it's the sort of old style populism that has its attraction. In Washington, we have this tendency to focus on the indicators and call it a mild recession.... That makes it all sound painless. But some areas are hurting badly." A number of experts say Harkin is well positioned in this Democratic race, which also includes former Sen. Paul Tsongas of Massachusetts and Gov. L. Douglas Wilder of Virginia. Several other Democrats may soon enter the campaign as well. They include Gov. Bill Clinton of Arkansas, Sen. Bob Kerrey of Nebraska, former Gov. Jerry Brown of California, and Rep. Dave McCurdy of Oklahoma. But insiders say that Harkin, armed with labor union money and manpower, is "the one to watch" in the next couple of months.