HE IS A FRIENDLY, apple-cheeked farmer from Michigan, with a conservative voting record, close-set eyes and a diamond ring. He has Norman Rockwell reproductions on the walls of his Capitol Hill office and a picture of Ronald Reagan in his gallery of the last six presidents. When he says "filched" as in "filched documents," it comes out "flitched."

He is Rep. Donald Albosta, a 57-year-old Democrat and former Michigan state legislator, who would not have been his colleagues' first choice to run an investigation of the covert side of the Reagan campaign. In fact, his name might never have shown up on a candidates' list, judging from the off-the-record comments heard last week. "Not very penetrating" and "not up to the job" was the prevailing tone.

"God moves in mysterious ways," sighed a Democratic leader, on hearing that Albosta, despite the cold water dumped on him by the House Speaker Tip O'Neill, planned to go forward with his inquiry into the making of President Reagan.

Albosta's wife, Dorothy, who works in his office as a volunteer, is used, although not resigned, to hearing putdowns of her husband. "I know the 'dumb farmer' label sticks. That's just because people never saw Don when he was doing the PBB (the toxic substance found in cattle feed) back home. He stood up to the speaker of the House, the governor of the state and the Farm Bureau. He won out because he was right. They'll find out."

Albosta chose himself to look into the curious story of how the Carter debate briefing papers -- "Did they have legs?" -- got to the Reagan camp. His two sharp young aides on the human resources subcommittee, 25-year- old staff director Micah Green and 32-year- old counsel John Fitzgerald, spotted the little item in the papers about "Gambling with History," in which Time correspsondent Laurence Barrett wrote about a mole who had filched Carter's debate papers for the Reagan team.

Since Albosta's subcommittee is rewriting the Ethics in Government Act, they brought it to his attention. Thereupon they wrote identical, civil letters to the parties involved: White House Communications Director David Gergen, White House Chief of Staff James A. Baker III, CIA Director William J. Casey and Budget Director David Stockman.

The quality of the replies was strikingly strained. They featured full disclosures to questions that had not been asked. Super- smoothie Baker, for instance, cited his record as a clean campaigner in previous outings. Imperturbable Gergen unaccountably argued that the "insignificance" of the documents, which he did not remember in any case, was proven by the fact that Time had not seen fit to print a line about them. Casey, who keeps stock prices in his head, gave a kind of unnerving reassurance that the nation's secrets are safe with him -- he remembered nothing.

The return letters brought Albosta the full- celebrity treatment. He said he didn't suspect anybody but thought that the cynicism of the nation, as reflected in non-voting patterns, would only be increased by a failure to look into things.

Suddenly Gergen discovered in his files some 700 pages of Carter debate material, and wrote a letter of apology to Albosta.

The president had a press conference which was mostly marked by forced laughter. Albosta had a press conference and announced plans to proceed with a "low-key" investigation.

The way things are developing, the congressman and his staff may have little to do. It may be enough to step aside and let Stockman, who served with Albosta in the House as a congressman from Michigan, do the talking.

Kojak would call Stockman a "motor mouth." He confides in strangers. He has a compulsion to let people know that he knows more than the people around him. What other motivation could he have had for his "Trojan horse" indiscretion about Reaganomics to William Greider.

For that, Stockman got a trip to the presidential woodshed. His latest revelations, however, seem to guarantee that he will soon become a figure in the unemployment statistics. On the day of the debate, Oct. 28, 1980, Stockman went home and bragged about playing Carter in debate rehearsals, working from "pilfered documents."

A reporter from the Elkhart, Ind., Truth, faithfully recorded it all and the story resurfaced on the wires last Wednesday.

Stockman's boss is now in the unenviable position of having to convince the country that nobody told him what Stockman told the Optimists Club of Casapolis, Mich.

Albosta, who is being reappraised by his colleagues in the light of what he has wrought with a single letter, will not hold public hearings any time soon. He may never need to.