"I reached the point where I was dating checks for the year 1939."

That would have been 16 years before Michale R. Beschloss was born. But at the time of the misdated checks, Beschloss was undergoing total immersion in the 1930s and '40s, absorbing the flavor of those years through old Paramount newsreels, newspapers, magazines and memoirs while doing research on his undergraduate senior thesis.

Those years came live to him, and he brings them alive in a new book, "Kennedy and Roosevelt: The Uneasy Alliance." It is a study of the personal and political relationship of two powerful and ambitious men who have figured significantly in American politics both by their own deeds and by family legacy. Now, at 24, Beschloss has seen his undergraduate thesis ("hardly a sentence survives") grow into a book that became the lead offering on this spring's lost of a major publishing house. "Kennedy and Roosevelt" has drawn praise from reviewers for both its meticulous history and stylish writing.

Why did the study of the relationship of Joseph P. Kennedy and Franklin D. Roosevelt evade a historian's attention until an undergraduate student at Williams College began work on a thesis?

As Beschloss tells it, he was in the right place at the right time. He was studying at Williams under historian James MacGregor Burns, and documents became available in both the United States and Britain.

"British Foreign Office documents were released after 35 or 40 years. What a delight to do such research," Beschloss recalls. "In spidery handwriting, British diplomats carried on the old tradition of reporting diplomatic gossip. Later, a colleague would come along and comment: 'Quite so. Isn't that awful? The dialogue was charming."

Beschloss feels that the Kennedy-roosevelt alliance offers the opportunity for an arresting examination of leadership.

Here were two men of wealth, power and persuasion in a love-hate relationship. They swam, sailed and drank together and yet called each other sons of bitches in private to friends. Both tried to manipulate and use each other.

In his book, Beschloss is at his best when he portrays Kennedy and Roosevelt as two men of contrasting traditions with conflicting views of public service and leadership. Kennedy, son of a ward heeler and tavernkeeper, was practiced in the ward politics of East Boston; Roosevelt, patrician of Hyde Park, was nurtured in noblesse oblige politics of the wealthy.

The examination of the Kennedy-Roosevelt relationship, uneasy at best and bitter at worst offers present-day parallels, Beschloss emphasizes.

"It gives us insight in what happens in a president's administration," he points out. "On the surface, we see people working together and programs coming out. But underneath is jockeying, conflicting egos, different public visions.

"Look at the Vance-Brzezinski clash today in Carter's adminstration. Ickes used to run to FDR to tell on Kennedy and vie for the president's favor."

Sounds like a subject for another historical examination and book?

"I don't need much encouragement," smiles the 24-year-old author.

A self-assured young man with a likable smile, Beschloss has a consumming curiosity that turns an interview into a two-way exchange of questions. Yesterday, sitting in the dining room of the Sheraton-Carlton, history intruded on the present for him.

"This was the setting for a wonderful Kennedy anecdote," he recalls. "In June 1938, Kennedy went to Hyde Park to talk to FDR. It was amiable and relaxed. But Roosevelt had Stephen Early leak a story to Walter Rohan of The Chicago Tribune that the president was furious with Kennedy.

"There was a headline about 'the chilling shadow cast over friendship.' The next night, in this room, Kennedy came to a reception for the media and, seeing Trohan across the room, good-naturedly drew his forefinger across his throat."

It is Beschloss' conclusion that leadership emerges from conflict as well as cooperation.

"In the end Kennedy and Roosevelt were more effective than they would have been on their own," he says.

Last night there was a small gathering of surviving New Dealers who lived through the turbulent, pivotal era of American politics covered in Beschloss' book. They came to a reception in the author's honor given at the Cosmos Club by his publisher, W. W. Norton. And in a turnabout of the usual autograph-party practice, it was the 24-year-old author who asked Benjamin V. Cohen and Tommy (the Cork) Corcoran to autography a copy of the book he has written.

Cohen, a member of Roosevelt's brain trust, was the author of legislation that set up the Securities and Exchange Commission. Kennedy who had expected to be named Roosevelt's Treasury secretary, instead was passed over for a Cabinet post and was named to head the SEC. The successful Wall Street speculator of the '20s became an effective government watchdog of the securities industry.

Corcoran, the political bon vivant, remembered the fun of the Roosevelt years.

"FDR loved to sing," he told Beschloss, who tells in his book of the night the president called Kennedy and invited himself to an impromptu dinner at Kennedy's estate on the Potomac. Arthur Krock, the political correspondent of The New York Times, who had been visiting, was singing below.

Also attending the reception last night was Benjamin Welles, who is working on the memoirs of his father, Summer Welles, under secretary of state during the Roosevelt era. Welles was Kennedy's ally in the State Department during those days when Kennedy, the first Irish-Catholic ambassador to the Court of St. James and a non-interventionist, was being bypassed by FDR in his dealings with Britain.

About halfway through the two-hour reception, Cororan confided, "Ben (Cohen) and I are going to evaporate. He has to go see his dog and I have to see my girl."