THE NEW IDEA may very well be the oldest idea in politics.

In 1946, John F. Kennedy '40's slogan in his first Congressional race was, "A new generation of leadership."

In 1960, John Kennedy said on the Presidential campaign trail in Warm Springs, Ga., "A new generation of Americans has assumed leadership--a new generation of Americans that is not satisfied to be second best."

In Wisconsin the following week: "I think we must move. I think we must push the United States ahead again. I think we must give this country new leadership. I think America must move forward again."

In Washington, D.C.: "This is a critical hour in our nation's history."

In Akron, Ohio: "This contest is between those who say, 'You never had it so good,' and those who say, 'This country can do better.'"

And in various places, on too many dates to mention, Kennedy called for "new ideas and new leadership."

In a debate among the Democratic Presidential candidates in Des Moines, Iowa this year, Sen. Gary W. Hart (D-Colo) said: "We used to have Democratic Presidents who asked us what we could do for our country,...who challenged us to express our idealism.... I think this country can do better...We are at a critical time in this nation's history. This is an election about our future versus our past...I am running and I need your help to move this country into the future. I ask you to help me move this country forward."

And at a debate in Hanover, N.H.--as well as at campaign stops all over the country--Hart called for "new ideas and new leadership...a new generation of leadership...[for] a new generation of Americans."

In 1960, John Kennedy was lucky. His New Idea rhetoric carried him past older and poorer rivals for the Democratic nomination, and then just barely past Vice-President Richard Nixon for the Presidency. Twenty-four years later, though, Gary Hart won't be so lucky if he depends upon New Idea rhetoric alone to win the Democratic nomination, and then the Presidency. In the rosiest of scenarios, Hart might overwhelm Walter F. Mondale simply by convincing voters that the former Vice President is a figure from the discredited past. He would then go on to win the nomination, and, course uncorrected, lose the general election.

For Ronald Reagan--who will defend a record of his own making--is not Vice-President Richard Nixon. And Gary Hart--who has not yet demonstrated any great personal charisma--is not Senator John Kennedy. More important, Ronald Reagan offers a clear--albeit atavistic--vision of what he would like America to be Reagan is dead set on dismantling government--or at least the kind Americans have known for the past forty years. His American future is a blurry dream of what this country never was in the nineteenth century.

He has opposed progressive taxation an un-American. He has opposed government regulation as intrusive. He has opposed every arms-control agreement to date.

He believes children should pray in public schools. He believes women should be denied the right to terminate their pregnancy. He believes that military superiority should be our goal. Ronald Reagan's vision of the future is a cross between Pollyanna and Darwin.

In the Democratic Party, Walter Mondale offers no new vision, and for that reason, more than any other, the American voter will not choose him in 1984. Mondale is in favor of the Democratic innovations of the past forty years, and so are most Americans. Mondale is against the Reagan regression of the past four years, and so are many Americans.

Well, that would seem enough: be for something a lot of people are for, and be against something a lot of people are against. But, unfortunately, there's a third requirement: one need explain in clear and inspiring terms just where we should go from here. And it is on this count that Walter Mondale fails and Gary Hart must succeed if he is to continue to establish himself as the alternative to Mondale.

TO WIN the Democratic nomination and then the Presidency, Gary Hart must infuse his Kenedyesqsue rhetoric with Rooseveltian innovation. He must stop talking about a "new generation of leadership," and start explaining what bold, new, creative solutions he can offer. As Ronald Reagan is the philosophical descendant of Barry Goldwater, Gary Hart can be the heir of Camelot. Hart has cast himself as a progressive liberal; now he must offer--as Goldwater called it in 1964--"a choice, and not an echo."

So far, it seems, Hart has unveiled only one new idea, along with some vague new themes. His Individual Training Account--to be contributed to by both employer and employee--would be used to retrain workers in traditional and declining industries. It's a good idea, but it should be one of many, not the only one. Military reform and industrial policy are two themes Hart has often mentioned, but rarely explained. And neither is straightforward enough to evoke passion--much less interest.

Hart should announce a comprehensive plan of action to revive the economy. Why not set up a venture-capital agency to help start-up companies? Assemble twenty top corporate leaders to assess industrial prospects, and then let them choose high-growth sectors to invest in. Japan and West Germany have set up such programs with success.

On the international trade front, declare that the United States will match all its trading partners' import restrictions. We might be surprised how quickly their barriers go down if there's a threat that ours will go up.

Hart talks about "restoring compassion to our government...[and] providing new help to the people who most need the assistance of this nation...." Why not set a minimum level of income for all Americans and couple it with the so-called negative income tax? This tax would guarantee a minimum income without eliminating the incentive to become self-supporting. The plan has supporters on both sides of the political spectrum, including former Senator George S. McGovern and Nobel laureate Milton Friedman.

Nearly 70 percent of families below the poverty line are led by single women-Establish a national system of day-care centers to educate and feed poor children. Limited programs of this--Head Start is one--have been overwhelming successes.

Even the most prudent middle-class family can be devastated by unforseen medical bills. Every American should have access to immediate, safe, and affordable medical care. Hart should propose a program of national healthcare insurance.

Progress has been slow with environmental protection Try a different approach Virtually all professional economists favor taxing polluters per unit of discharge. Such a plan could save bilions of dollars, and, more important, provide unavoidable incentives not to pollute.

Hart talks about "restoring the fire of idealism" to public service. He should call for a program of national non-military service. In a kind of national Peace Corps, volunteers would help the poor and the homeless and the elderly all over the country. In return for their service, they would receive educational grants, thus ensuring that all Americans have the opportunity to go to college.

Hart correctly identifies that enemy south of the border as "hunger, poverty, and disease--not Communism." Why not propose a new Marshall Plan, but this time for South and Central America? We helped Western Europe after World War II not out of charity, but because we knew that a strong and independent Europe was essential to our economy and our security. So to with the Americas: help them develop into self-sufficient trading partners and allies, not industrial colonies.

THESE ARE new ideas in the progressive tradition: bold, creative, and experimental. But the path of progressive leadership through this century is still unfinished. It no doubt began with the New Freedom of Wood row Wilson, continued through Franklin D. Roosevelt '04's New Deal, and then the New Frontier of John Kennedy.

The election of 1984 may very well decide the future of progressive liberalism--and this country. Were the national vicissitudes of Vietnam, Watergate, and Ronald Reagan merely twists in the path? Or have we taken a different path altogether? To win the Presidency this year Gary Hart must craft a new vision; from his new rhetoric.