To his credit, Walter Mondale has been professional enough as a politician and objective enough as a presidential candidate to recognize what was possible to accomplish in the 1984 campaign -- and what was not.

The classic challenger's strategy was unavailable to him. He could not prove that the man in office -- President Reagan -- has failed to do what he set out to do, and therefore should be replaced. Reagan has achieved many of his initial objectives -- reducing taxes, inflation and the growth of domestic government, expanding and proican military and paramilitary power.

So Mondale has staked his claim for the presidency on a different and more difficult proposition: an effort to persuade the voters that Reagan's program is inadequate or out of line with the nation's long-term interests.

In doing so, he has fallen back on the best of the Democratic tradition, which he embodies. From beginning to end, he has made the case for an activist government, headed by an engaged president, whose highest priorities are equality and social justice at home, arms control, peace and human rights abroad.

By resting his case on that bedrock Democratic tradition, Mondale -- win or lose -- has been true to his own principles. Whenever he has had his back to the wall -- in the challenge from Gary Hart in the primaries and in the closing days of this campaign -- he has responded as his mentor, Hubert Humphrey, would have wished, by reaffirming the validity of that political tradition.

For many of those voters, like myself, who were reared in the same tradition and who wish to see its principles maintained in Congress, the executive agencies and, especially, the Supreme Court, the choice in this election is very clear and very easy. It is no accident that Mondale runs best among us middle-aged voters who cast our first ballots when Adlai E. Stevenson or John F. Kennedy was running.

But the American voters are not all aging Stevenson-Kennedy fans, and an election is not just a choice between competing parties and traditions. It is also a choice between leaders and directions, and it is here that Mondale has shown persistent shortcomings.

One failing has been intellectual. When he left the vice presidency in 1981, he publicly acknowledged the need to rethink the policies and programs of the Democratic Party. If he tried to do that, he failed.

He was too wedded to the Democrats' traditional focus on Washington to grasp the power and attractiveness of the policy initiatives being taken by creative Democratic governors across America. He moved his home to the St. Paul suburbs, but his heart stayed on the Potomac. Instead of presenting himself as the ally the Rileys and Grahams and Babbitts an Lamms need in the White House to make their programs work, he allowed Reagan to become the candidate of renewed federalism -- and to monopolize the South and the West.

He was too wedded to the Democrats' traditional fiscal approach to recognize the power and attractiveness of the tax-simplification and economic-development ideas put forward by creative Democratic senators and representatives. Instead of presenting himself as the ally the Bradleys and Gephardts need in the White House to enact loophole-closing, rate-reducing revenue reforms, he fell into a fatal argument about tax hikes -- and let Reagan become the candidate of the younger voters eager for economic opportunity.

The second failing has been political. Mondale is an "insider" candidate, enjoying personal friendships and political alliances with leading politicians and interest-group leaders, who would be a real asset to him in moving his program as president.

But these folks are, in most instances, no longer able to move their own constituencies. Whether from a life-long habit of deference to his political sponsors or an exaggerated personal reticence, Mondale has relied too much on what they can deliver -- and too rarely has tried to mobilize those constituencies for himself.

The result has been doubly damaging to his standing as a potential president. He has failed to build a solid, enthusiastic personal following that would rally to him in the hard times any candidate or president must endure. And in his dealing with those other leaders, he has accepted far too much of their own agendas as his own, to the detriment of the independence and strength that voters hope to see in their president.

Mondale has appeared only fleetingly as his own man. Most of the time he has been seen behind the screen of Tip O'Neill and the Washington power brokers, Lane Kirkland and organized labor, Jesse Jackson and the blacks, Geraldine Ferraro and the feminists.

When people have seen him plain -- as in the Louisville debate -- they have sometimes been pleasantly surprised at what an effective, attractive and decent politician he is.

If he does become president, the nation will have to hope that Mondale will be more open to fresh ideas and people and more confident of his own constituency-building abilities than he has been as a candidate.

If he loses, as seems likely, his honorable but flawed effort can provide important lessons for the next generation of Democratic leaders, who must find ways to renew the appeal of the liberal tradition.