Alfred Mossman Landon, who will be 100 tomorrow, was the quintessential American political fall guy when he was the Republican candidate for president in 1936. He was flattened by Franklin D. Roosevelt's second-term steamroller, much as was George McGovern by Richard Nixon's and Walter Mondale by Ronald Reagan's. Landon did worst of all: he carried only Maine and Vermont with eight electoral votes, exactly as Democratic national chairman Jim Farley had predicted. McGovern got 17 electoral votes, Mondale 13.

Now all this is history, but what makes Landon special for me, aside from my regard for his daughter, Sen. Nancy Landon Kassebaum, who is from her father's home state of Kansas, is the reportorial brushes I had with Landon 51 years ago. And perhaps there are some lessons, too, in the '36 outcome.

On Aug. 22 of that year, when I was between jobs, I went to the small western Pennsylvania town of West Middlesex to hear Landon make his opening pitch in the Keystone State, which had stuck with Herbert Hoover in '32 against FDR. The town was Landon's birthplace, but aside from some GOP homilies about maple syrup and other small-town virtues, a sentence I wrote that day in a free-lance story still has resonance: ''Of late the Negro voters have been weaned away from the elephant by Democratic patronage and by an Equal Rights Bill, passed by the Democratic State Administration, imposing heavy penalties for racial discrimination in public places.''

In 1936 the GOP had a dilemma: finding a nominee who could beat ''that man.'' Landon was no GOP dinosaur; he had voted for Progressive Robert M. LaFollette in 1924 against Mr. GOP himself, Calvin Coolidge. For this and other party infractions, some considered him a ''me-too Republican,'' a derisive label long to be hung on the party's liberal wing, because Landon conceded that the New Deal had done some useful things.

In Kansas, Landon had won the governorship against the Democratic tides of 1932 and 1934, and he represented the reform wing of his state party. Early on, the Kansas City Star's Roy Roberts started plugging Landon for president, emphasizing his strong opposition to FDR's deficit spending. The new owner of The Washington Post, Eugene Meyer, and his wife Agnes by now were disenchanted with FDR's ''fiscal irresponsibility,'' and they found Landon an attractive alternative. The Post became a major backer in spreading the word in the East.

At age 48, Landon was nominated on the first ballot at the June convention in Cleveland. His running mate was Chicago Daily News publisher Frank Knox.

For a while it looked as if he truly might have a chance after the first post-conventions Gallup poll gave FDR only 51.8 percent to Landon's 48.2, with an electoral-vote majority for the Republican. The Post editorially foresaw ''the closest election since 1916.''

But scientific polling then was an infant industry, and GOP politicians of any ability knew different. And so they tried to frighten the voters as Election Day neared with the specter of Social Security. In Toledo, I reported, they began passing out official-looking notices (and tucking them into pay envelopes of blue-collar workers) announcing in ominous words coming ''pay deductions'' because the new system would go into effect next Jan. 1. GOP national chairman J. D. M. Hamilton charged that the New Deal intended to put dog tags around the necks of the 25 million Americans soon to be covered. In the end Gallup did predict a big FDR victory, but the Literary Digest, long an influential wide-circulation magazine, committed publishing suicide by predicting a sweeping Landon win.

As the campaign came to a climax, Landon swung through Toledo, where I had gotten a job as a reporter for the News-Bee (a paper that survived for only two more years). The candidate left his train for a motorcade, and thanks to a News-Bee photographer I have a record of the occasion: a photo of Landon in his convertible and of me, the skeptical kid reporter of 25, standing among others at one side. Some three decades later when I was at The Post I sent the print to the governor in Kansas, asking for an autograph. He proved that the FDR landslide had not deprived him of his sense of humor: ''For Chalmers M. Roberts,'' he wrote, ''his first presidential campaign and he got to Washington. With best wishes, Alf M. Landon.''

On election night the streets of Toledo were thronged as citizens snatched up newspaper extras, though radios by then were common. FDR carried Ohio this time by more than 600,000 votes; nationally he won 60.8 percent of the popular vote to Landon's 36.5, with fragments for minor candidates.

It was back to Kansas for Alf Landon, and he's been there ever since, horseback riding well past his 90th birthday, a cheerful and unembittered Republican. The only thing he couldn't believe, his daughter told me a while back, was that Kansas would ever elect a woman to the Senate, least of all his own daughter.

The political lesson of 1936, however, really has more to do with FDR's second-term landslide than with Landon, the victim. FDR misinterpreted his huge ''mandate'' as a ''go'' for his plan to ''pack'' the Supreme Court, that body of "nine old men," who had been striking down his first-term social legislation as unconstitutional. It was his worst political error, an exercise in the arrogance of power, and compounded by his ''cover story'' that more justices were needed because of the heavy burden of court business.

The whole second-term business sort of reminds me of Richard Nixon's Watergate cover-up after his big ''mandate'' and, once more, of Ronald Reagan's use of his ''mandate'' to do what he wanted to do, public opinion and the Constitution to one side, in what became the Iran-contra affair.

Maybe we ought to institute a ''verdict'' vote at the end of a second presidential term, a sort of ''satisfactory'' or ''unsatisfactory'' pair of boxes to be checked by the voters as they elect his successor. It's the only way I can think of to check the excesses of those ''mandates.'' I wonder if Alf Landon wouldn't like the idea.