In a column by Mark Shields June 18, it was incorrectly reported that Sen. John Glenn shot down 20 MiGs during the Korean War. The correct figure is three. We regret the error.

Pablo Picasso, never an especially big booster of things and persons American, said of him: "I am as proud of him as if he were my brother." That was 1962. Since then his fellow citizens have expressed their own pride by naming schools, streets and babies after him. Many are unaware that he was a Marine pilot who, in Korea, shot down some 20 MiGs. But virtually everyone knows John Glenn, genuine American hero. Now in his second term, Sen. Glenn may soon become a candidate for the 1984 Democratic presidential nomination, a possibility that excites some Democrats and terrifies a few Republicans.

Just about four years ago, Jimmy Carter's political strategist was scared stiff about running against Sen. Howard Baker in 1980. Baker, a "thoughtful moderate" with demonstrated appeal to Democrats and independents, was properly viewed as the strongest Republican opponent in a general election. Baker's problem, like that of so many moderates in a multi-candidate field, was to be the primaries. Thoughtful moderation, as a rule, does not produce political true believers. John Glenn could be either the 1984 Democratic Howard Baker or the 1984 John Kennedy.

Last week over lunch in his Senate office, Glenn talked about the presidency and about running for that office. Politically, his strengths and weaknesses are available for viewing in Ohio vote returns. In November 1974, Glenn carried all 88 Ohio counties in effectively ending the political career of his opponent, the Republican mayor of Cleveland. In November 1980, Jimmy Carter lost 78 Ohio counties and the state by 450,000 votes; Glenn lost one county and won re- election by 1.6 million votes. His primary record is not quite so impressive. In 1970, he lost a big lead and the Senate nomination to liberal Howard Metzenbaum, a Cleveland millionaire. In 1974, by a 54-to-46 percent margin, he defeated Metzenbaum, who had been appointed to the Senate six months earlier.

Glenn's interests in issues--strengthening government research and development, a stop to nuclear proliferation, improved aid to higher education (all of which he pursues with characteristic determination) --are not calculated to yank audiences to their feet. But, on government's positive contributions in agricultural and industrial research, Glenn is very emphatic. He criticizes the administration for "backing into the future." Of today's economic casualties, he says: "where the defenseless are concerned, compassion is at least as great a virtue as frugality."

Glenn is bullish about America's future. One North Carolina Democrat, after looking over the candidate field and watching Glenn sign hundreds of autographs at a state party dinner, is guardedly bullish on Glenn. "Wouldn't it be great to have help, instead of a drag, at the top of the ticket?" he mused, only to add that the Mondale staff follow-up was quite superior to Glenn's.

In 1984, the Democrats, whose nominees have only once won a majority of the national vote since FDR, may pass over John Glenn just as the Republicans passed over Howard Baker. If that happens shed no tears for John Glenn. He is not a totally accomplished politician, which could mean he might not be a very effective president. But John Glenn, unlike some presidential candidates and a few recent presidents, knows who he is. He's "comfortable" with himself.