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.