There it was on the front pages of the Sunday papers of the nation. For any Jimmy Carter strategist, it was the luck break that campaign managers forever dream about, but discipline themselves never to rely upon.

It began with a campaign photo opportunity. Sometime on a hot California Saturday, Ronald Reagan, 69-year-old presidential nominee and amateur botanist, was pruning some bushes on his property. Reagan, who looks like a pre-inflation million in his work shirts and jeans, chose in the presence of working photographers to remove his work shirt. Without it, he looks like a lot less than a million. He looks like 70.

What Reagan, and whoever scheduled and directed the event had done, was to raise, through a perverse form of voluntary indecent exposure, the dreaded age issue -- and to raise it more dramatically and graphically than Carter and the Democrats would have ever dared to do.

But by Monday it had passed; there were the stories about the president's brother and the first family's involvement in the nation's foreign policy. With three months to tgo until Election Day, Jimmy Carter is squarely on the defensive, in trouble and face-to-face with an immutable political truth: everything in a campaign is a poll. Carter is hearing daily, from the men and women who earn their living from politics and election that he is a liability, a drain. More than a few Democrats are approaching their New York convention with all the joyless inevitability of the political equivalent of Jonestown.

Any incumbent has available two routes to reelection: to trumpet the achievements of his previous term and/or to portray his opponent as someone fearsome and even loathsome, someone who would threaten your safety through his actions or ignorance. self-evident reasons, the 1980 Carter campaign has settled on the second approach.

These two weeks before the opening of the New York convention were supposed to be the beginning of the Carter offensive. Instead, junior Democrats in Congress are urging a Cabinet member and the vice president to run for president. The administration and the campaign are rocked daily by disclosures about Billy Carter's access and activities. And the president himself is especially hurt by a story that he forgot a meeting with the attorney general of the United States.

Anybody who has ever held a political office above that of library trustee knows that you do not forget meeting with the DA, especially when the DA is discussing whether or not your brother is to about to be indicted. A pol may honestly fail to remember the minutes of a meeting with a prince, a prime minister or even a pope. But nobody who has ever worried about absentee ballots or inquired about an asphalt contract ever forgets any conversation with any prosecutor.

Jimmy Carter is hearing the muffled sounds of mutiny from the delegations of New York, Illinois, Ohio, and Pennsylvania. These are the worried words of men who need to preserve majorities in legislative districts and now see that task made more difficult by the Lib-Fibs. These are not the true Carter loyalists on whose couches Candidate Jimmy slept. These are concerned professionals for whom politic is important and patronage is not to be patronized.

When the Democratic convention does end, only 11 weeks will remain in which to engage Reagan, to change his status from that of Critic/Alternative to the Hand on the Button. As much as Jimmy Carter did, Ronald Reagan benefited from the Iranian events this year, because Reagan and the expected press scrutiny of him were also pushed off the front pages.

It is hard to imagine that Ronald Reagan who has given the distinct impression that he genuinely believes there is more oil under second base at Yankee Stadium than the NATO countries have used since VJ Day, will not be thoroughly challenged during this campaign. But because of deposit slips and cables and disclosures, Jimmy Carter could fast be approaching that most uncomfortable of political positions: where all you can do is try to provoke your opponent into the Giant Mistake. Candidates who do this usually come off as grating, and the longer it goes on, the bigger the mistake that is required. The Democrats, a party lacking both a defined mission and clear vision, are in big trouble.