SOMETIME in his formative years -- my guess would be 10th grade -- Sen. William S. Cohen (R-Me) recieved a regrettable piece of advice. Somebody -- probably a doting English teacher -- told him he had the makings of a poet. The awareness of his gift has gravely impaired Cohen's ability to communicate in English ever since.

The conclusion follows from reading -- to be blunt about it, wallowing through -- the senator's stupendously overwriten new book, Roll Call. It is a journal of Senate life, but it reads more like an anthology of every "poetic" cliche Cohen ever heard -- plus a few original lyric flights of his own.

In just the first half-page, for example, we find such hoary favorites as "yellowed pages of yesteryear," "firey birth as a nation," and "riptide rush of events," together with the genuinely original assertion that "there is more than one exit lane to the boneyard of indifference."

Later on, when he really gets warmed up, our poet recalls an election night when he faced "the dark swamp of defeat." "By this time," he writes, "that gray moth of doubt in my mind had ballooned into a terrifying pterodactyl."

Even for a senator, this is a major-league mataphor mixing. Any reader of the Congressional Record knows that dark swamps and fiery births are standard items in the senatorial lexicon. But a man with ballooning pterodactyls on his mind represents a whole new dimension. In Cohen's case, the extra dimension is "poetry."

The senator has been an amateur poet most of his adult life. In the late 1970s, when he was a congressman, his poetry began appearing in the Congressional Record . One thing led to another, and in 1978 he published a volume of poems titled Of Sons and Seasons. This new book is a more typical congressional venture -- every few years some member publishes a "daily journal" of life on the Hill -- but Cohen seems determined to remind the reader that he is a real live poet. The result is a labored, mannered prose so annoying (to this reader, anyway) that it continually draws attention fromthe substance of the book.

The traditional adjective used to advertise insider accounts of Congress is "candid." Sure enough, Simon and Schuster is touting Cohen's book as "remarkably candid." This is partly true.

Cohen, 40, is known on Capitol Hill as a bright, hard-working, smooth-talking moderate -- the very model of the modern media-oriented senator. But he is honest enough to admit here that even the model has sub-par moments. On one roll call, Cohen gets so confused by the floor debate that he ends up voting for an amendment he strongly opposes. On another, GOP senators delay the vote for no reason except to aggravate the Democrats. At a committee hearing, Cohen's questions are so irrelevant his staff passes him a note telling him, in essence, to shut up.

But all this candor has its limits.

Cohen complains repeatedly about his busy travel schedule. Sourly, he recalls a trip to St. Louis for a speech and press conference. The audience was "aggressive", he says, and the reporters rude. The burdens of public life! As he boards his plane for Washington, he muses about "what you'd find if we started peeling back the layers inside our skulls to find out why we do this."

One reason senators do things like that is that they earn good money for it. According to disclosure forms he filed last year, Cohen was paid $2,000, plus expenses, for his morning in St. Louis. That datum, unmentioned in this "remarably candid" book, may or may not explain why Cohen made the trip. But it does offer a contrasting gloss on his lament about the sorrows of a traveling senator.

Still, Cohen's book offers some insights into a senator's way of life.

It is evident from this journal that the senator from Maine -- like many other senators -- keeps his home, head and heart in Washington. Early in the book his father visits D.C. "to see where I now lived and worked." His children's schools, friends and soccer leagues are here. Father and sons go on vacation -- to Rehoboth Beach. The Senator visits Maine several times in the course of this book, but his family rarely sees the state he represents. Once when he plans to take his wife along, he finds he cannot afford her air fare. She stays home -- i.e., Washington.

Similarily, Washington issues are much more attactive to the up-and-coming senator than local concerns. "I was annoyed that I had to break away from the Armed Service Committee to discuss the programs and policies that a 'rural center' had to offer," he writes. Further on, he is in the midst of a committee session on the MX missile when he learns of a meeting on shoe imports, a key issue for Maine. He leaves the MX discussion temporarily, but only because "my staff insisted."

Cohen notes that "it is difficlut to keep one's self in persepective" in the Senate, and his book vividly demonstrates the striking self-esteem that seems characteristic of people who become senators. "My speech was received enthusiastically," Cohen writes at one point. Again: "I recited the poem I wrote . . . they enjoyed ti very much." Again: "the audience broke into applause. I left on a wave of emotion and good will." What kind of person says that kind of thing about himself?

Cohen's book, an all-purpose storehouse of cliches, has a lot of answers for that one: The kind of person who has "sacrificed upon the bloody and brutal altar of public service." A person skilled at "running the banner of moral outrage up the flagpole." A person who thinks he is "in control of the whitecapped wave of world events." A person full of ideas that have "bloomed like a geranium in the flower box of my thoughts." And on and on, through the yellowed pages, straight to the boneyard.