One hallmark of Washington: It takes only seconds for a Question to become a Concern, a Concern a Fear, a Fear an Issue. Just yesterday, the once-again popular Age Issue spawned the new and exciting Makeup Issue.

It all started in the morning, when President Reagan decided to explain something to reporters as he left the White House for a one-day campaign trip to Michigan. Although some observers had said he looked weary during Sunday's debate with opponent Walter Mondale, Reagan said he "wasn't tired."

"With regard to the age issue," he added, "if I had as much makeup on as he Mondale did, I'd have looked younger too."

A reporter asked, "You didn't wear any makeup?"

"No," Reagan replied, "I never did wear it, even when I was in pictures."

Sometimes an Issue is asking for an answer.

"This morning, the president said the problem in the debate was makeup," Mondale said at an appearance in Pittsburgh. "That's the same answer Nixon gave.

"Mr. President, the problem isn't makeup on the face -- it's the makeup on those answers that gave you problems. Yesterday, he wanted to arm-wrestle me. Well, we had a little brain wrestle on Sunday night, didn't we?"

For those with short memories: Among debate aficionados, Richard Nixon is believed to have lost his edge in a 1960 debate against John Kennedy when his perspiration caused the powder covering his five o'clock shadow to run.

"All I remember is that Kennedy arrived looking tanned and fit. Nixon had been in the hospital with staphylococcus," said Don Hewitt, executive producer of CBS' "60 Minutes" and director/producer of the first Kennedy-Nixon debate. Hewitt offered the two men his makeup artist Frances Arvold. "Kennedy said no, he was tan. Nixon heard Kennedy say no, so he said no." Instead, he opted for a bit of light powder.

The issue came up four years later at the 1964 Republican National Convention.

"I was sitting with Richard Nixon," said Hewitt, "while Frances was making him up before he introduced Barry Goldwater. I said to him, 'You know, Mr. Nixon, if you let Frannie make you up back then, Barry Goldwater would be going out to introduce you,' and he said, 'You're probably right.' "

Reagan's citing of the Makeup Issue has delighted Mondale supporters.

"I think it's one of the great lines of the campaign," said Richard C. Leone, a political adviser with the Mondale campaign in charge of getting out the Mondale message.

"I think it's probably a very revealing statement about what he thinks is important," said Leone. "If you always judge things by appearance and not substance, even when you don't do well, you assume it's due to appearance, not substance.

"It means he hasn't learned the lesson of the debate, which is that sooner or later, it's what you're saying that matters, not only how you say it and how you look when you say it."

The first hint that the offensive against the Age Issue was branching out came Tuesday, when Maureen Reagan commented that both debaters looked tired by the debate's end and that "Mondale's makeup was beginning to drip."

The White House, which had relocated yesterday to Michigan with the president, didn't have much to say about the Makeup Issue, except to answer some basic questions.

"We've gotten two or three calls to see if that included light powder, because everyone knows he doesn't wear the heavy pancake," said White House assistant press secretary Anson Franklin.

It includes light powder.

"I am told he doesn't use anything," said Franklin. "He just doesn't want to use it."

And no, Reagan will probably not be stocking up on Max Factor for his second debate on Oct. 21.

"Knowing him," said Franklin, "I don't expect any changes in his practices."

In the camp of Mondale running mate Geraldine Ferraro, one staff member said their candidate had not yet discussed what makeup she will wear when she debates Vice President George Bush tonight.

From the Bush camp comes word that he will follow his usual technique.

"We haven't really talked about it but my assumption is that he probably will wear a little powder -- just to keep the shine down," said Shirley Green, Bush's deputy press secretary. "I'm snowed over the whole thing. Wouldn't you like to look good enough so you didn't have to wear makeup -- on or off television?"