Former Truman adviser Clifford says Mondale's still in the race

''It's still a race.'' So says a Democrat, but a Democrat of such distinguished credentials and long service to his country that his words are listened to with more than casual interest. He is Clark Clifford, an adviser to Harry Truman and John F. Kennedy, secretary of defense under Lyndon B. Johnson, a diplomatic emissary under Jimmy Carter - and one of the country's most eloquent Democratic spokesmen.

Despite Walter Mondale's present struggles and poor standing in the polls, he ''has an appreciable chance to prevail,'' Mr. Clifford told reporters at a recent breakfast meeting. It is possible, he suggested, that the people will begin to pay attention to the real issues instead of the Hollywoodized ''air of unreality'' that prevails in the current administration.

It is also possible, he said, that, as President Reagan campaigns, he will begin to make more mistakes, such as what Clifford called his ''weird' comment following the recent Beirut bombing when he likened the lapse in US Embassy security to the difficulties of renovating a kitchen.

Is this just a vain Democratic hope?

A parallel is drawn to the campaign of 1948, when polls showed Thomas E. Dewey so far ahead that ''there was no more need for any more polls.'' Clifford, who was traveling then on the hustings with Harry Truman in what was a ''discouraging'' climate, recalled for reporters a Newsweek poll of the country's 50 leading political experts that showed zero for Truman and 50 for Dewey.

''I never had a bit of confidence in anyone of those fellows!'' Truman was quoted as saying when shown the magazine.

Then, six weeks before the election, Clifford said, the Truman campaign party began to notice bigger and more enthusiastic crowds. The outcome is history.

Mondale is not a Harry Truman, Clifford acknowledged. But reputations grow and develop through the years. Truman at the time, he said, was a lone individual battling for his political life, talking about issues while Dewey was avoiding the issues and planning his future Cabinet. Today, too, an ''underdog psychology'' could come into play, said Clifford.

Seeing events against the backdrop of history, the courtly Democrat credits President Reagan with providing leadership at a time of national discouragement over the Vietnam war, the wounding experience of Watergate, and the seizure of American hostages in Iran. Reagan ''very intelligently sensed'' the deep discouragement and used it as he moved to restore the country's sense of confidence, he said.

''He has rendered a valuable service,'' commented Clifford.

But, he added, ''the price is much too high that we are paying for that service.''

There has been an ''incredible deterioration'' in the US-Soviet relationship that need not have happened, Clifford said. Reagan missed a ''signal opportunity'' to continue the policies of previous presidents in the arms control field.

''If Reagan had chosen to start negotiations right away with the Soviets, the most beneficial and benign results could have taken place,'' Clifford said.

What of the President's conciliatory speech at the UN and meeting at the White House with Soviet Foreign Minister Andrei Gromyko?

''This is no time to deplore it,'' said Clifford.But he said these were ''appropriate strategic moves'' with an election only a few weeks off that he doubts reflect a ''basic shift'' in the Reagan position.

He said there ''someday will be an accounting'' for the Reagan economic policies that have resulted in huge deficits. There is an ''air of unreality'' in the administration that charges the Democrats with ''doom and gloom'' and plays to the public's penchant for ''good news.'' That is ''exceedingly astute, '' said Clifford, but troubling for the welfare of the country.

Asked what other issues he would recommend Mondale exploit in the campaign, Clifford mentioned the administration's mining of the Nicaraguan harbors, which even Sen. Barry Goldwater decried as a violation of international law.

He also criticized Reagan's handling of the invasion of Grenada, which he characterized as a ''Hollywoodization of our government business,'' remarking on the fact that some 8,300 US military personnel were given medals for an operation against 600 Cubans.