DES MOINES -- A pair of political golden oldies who have largely dropped off the charts this year showed up in this temporary capital of presidential politics yesterday to demonstrate their timeless talent at striking a resonant chord in campaign season.

Still running after all these years, 80-year-old Harold Stassen, who first sought the Republican presidential nomination in 1948, called a news conference (and drew four reporters) to say he is the only GOP contender promoting the once-dominant policy structure known as "moderate Republicanism."

Not running this time, but admittedly envious of those who are, 1972 Democratic presidential nominee George McGovern gave a series of speeches and interviews here, drawing an affectionate response from Iowa Democrats who waited patiently to shake his hand and reminisce about the McGovern campaigns of 1972 and 1984.

McGovern said he was concerned that people might draw the wrong conclusion from a series of recent events: He was the guest at a political reporters' breakfast in Washington, D.C., where he seemed to hint that he might yet become a candidate this year; he has a long policy paper in the latest issue of the journal "Foreign Affairs," and he arrived in Iowa on the eve of the presidential caucuses Feb. 8.

"It all really is just total coincidence, hard as that is to believe," the 65-year-old McGovern said.

"I'm not going to be running in '88, tempting as it is," he said. "I consider it every four years. If it weren't for wiser friends of mine, I'd probably be in the race every four years."

Stassen, who has been in the race almost every four years -- seven times in all, each unsuccessful -- explained why he once again revived the perennial candidacy that makes him the butt of a thousand jokes. In doing so, he sounded a wistful note. "I want to get out and talk about the importance of the United Nations," Stassen said. "I was the youngest of the Americans who were there to draft the {U.N.} charter in 1945. Now, I am the only one living."

McGovern's day here, largely among Democratic friends, had the quality of a class reunion, but the former senator flashed his familiar fervor when he took on the subject of U.S. policy in Central America.

"It looks to me like {President} Reagan's doing a pretty good job of snowing the Democrats," McGovern told the Des Moines Chamber of Commerce. "If the Democratic Party had any guts or sense at all, it ought to stand up united against this {Nicaraguan} contra thing."

McGovern said it might be "presumptuous for a man who lost 49 states to advise the Democratic candidates," but he couldn't resist. "They ought to be telling the voters that the first thing they're going to do is change this unilateral policy in Central America," he said.

Together, Stassen and McGovern constitute living proof of the hypothesis that politics can serve as an elixir of youth. The octogenarian Stassen, his hair sandy brown and his voice deep and full-timbred, seemed as sharp and fit as the candidates who were not even born when he first played the game of presidential politics in the 1940s.

McGovern, his graying hair offset somewhat by the twinkle in his eye and his chipper, energetic demeanor, made no bones about the fact, as he put it, that "this business keeps you young."

"Don't let anybody kid you about running for president," the smiling McGovern told the Chamber of Commerce. "Running for president is far and away the best game in town."