Washington — There's an ironic, almost unnoticed twist of history as the 1984 presidential campaign enters its final weeks. Ronald Reagan, the Republican President, is invoking the names of Harry S. Truman, Franklin D. Roosevelt, Hubert H. Humphrey, John F. Kennedy, and other Democratic champions as he widens his appeal for votes.
Walter Mondale fumes. ''For a generation, my opponent fought Democrats tooth and nail,'' Mr. Mondale grumbles. ''Now, six weeks before the election, he lards his speeches with Roosevelt quotations. He gives a medal to Hubert Humphrey. He invokes Truman in Missouri. He invokes Kennedy in Connecticut. And he asks Democrats to become Republicans, as if it didn't matter.''
History, however, has something interesting to say about all this.
Look back at 1948. President Truman was battling for his political survival. Southern Democrats didn't like him. Northern liberals were cool to his candidacy. But there was at least one Hollywood actor who stood firmly behind Mr. Truman, despite the odds.
In a book by Ken Hechler, ''Working With Truman,'' is a photo of the beleaguered Truman speaking in '48 at Gilmore Stadium in Los Angeles. Just behind Truman is the earnest, young face of Mr. Reagan. Later, Reagan campaigned effectively on radio for the President.
Another book, ''Mondale,'' by Finlay Lewis, notes that in that same year, a young political figure in Minnesota, Walter Mondale, was unhappy about Truman's renomination. In fact, Mondale was so disenchanted, says Mr. Lewis, that he began wearing a button for William O. Douglas for president. Truman, it seems, wasn't the liberal hero that Mondale wanted, says Lewis.
All this is pertinent because President Reagan's high-horsepower campaign is now turning its engines in the direction of Democratic voters. During these closing weeks, Reagan overtures to Democrats - especially conservatives, Southerners, ethnic groups, and union members - will be a crescendo. To make these Democrats feel at home voting for a Republican, the names of Roosevelt, Truman, Kennedy, and Humphrey ring loudly at Reagan rallies.
Reagan - himself a former Democrat - reaches out to these voters by telling them that they have not abandoned the Democratic Party. Rather, the party has abandoned them, Reagan argues.
Mondale knows he must blunt Reagan's erosion of the Democratic base, or face the prospect of a crushing, McGovern-like defeat next month. He argues that Reagan is peddling a perverted version of history - one that Mondale says seems like the language of ''1984, the year of (George) Orwell and doublespeak.''
Mondale's Democratic Party, as he sees it, is one with a social mission. It sides with the workingman, as well as those unable to work. It sides with the sick and the homeless. It resists the overreaching power of wealth and corporate America.
Says Mondale: ''Millions of American children are born in poverty. Many go to school hungry. Many don't learn to read and don't learn to hope. And nearly everything we've done as a nation to help those children has been cut back by this administration....
''The Republicans say they're for family values. But families don't disown their weaker children. ... What would we say about parents who lived in high style and left their children in debt as a result?'' Mondale asks.
These are the Democratic values that Mondale says are still valid today and that are not reflected in the Reagan White House.
Republicans, in turn, argue that today's Democratic leaders forget that Roosevelt, Truman, Humphrey, and Kennedy stood for a vigorous foreign policy - one in which military power was used more actively than it would be by today's leading Democrats, such as Mondale or Sen. Gary Hart. Critics say current Democratic leadership is costing the party votes within its traditional base.
One reason Reagan's focus has turned to Democratic voters is that he's gotten just about all the mileage he can with Republicans and independents. A new ABC-TV poll finds Reagan currently supported by an overwhelming 96 percent of all Republican voters. He also gets nearly 2 out of every 3 independent voters. There is probably little room for growth left.
This leaves Democrats - especially Democrats who are disenchanted with the current nominee. Some of the prospects most likely to vote Republican include Southern whites, who have been scared by the Rev. Jesse Jackson's appeals to black voters; so-called Atari Democrats, the young, upscale voters who supported Gary Hart; ethnic groups, many of whom seem to like a strong, anti-Soviet foreign policy; and unionists, who are split about evenly between Reagan and Mondale.
The improved economy clearly helps Reagan with all these Democrats. But the President has also found issues that appeal to each specific group. Tuition tax credits for private and parochial schools, for example, are a strong attraction among traditionally Democratic Roman Catholic voters in the Northeast and Midwest. That may be one reason that in spite of Geraldine Ferraro's name on the Democratic ticket, Reagan is currently giving Mondale the boot among heavily Catholic Italian-Americans by a 66-to-34 margin.
One final point. Remember that medal that Reagan awarded recently to Humphrey? Some called it hypocrisy. But in 1948, when Humphrey was mayor of Minneapolis, Reagan went on the air to back him in his race for the United States Senate as a man ''fighting for all the principles advocated by President Truman.'' Such are the fascinating turns of history.