Theodore Rex. (Moose Tracking).

THEODORE REX by Edmund Morris Simon & Schuster, $35.00

A HUNDRED YEARS AGO, TO THE consternation and dismay of the conservative leadership of his own Republican party, Theodore Roosevelt became the 26th president of the United States. It proved a turning point in American history. The youngest and, by any reckoning, one of the most dynamic and fascinating men ever to reach the White House, Roosevelt--the Theodore Rex of Edmund Morris's highly readable new biography--followed a long succession of mediocrities who had presided over the country in the 36 years after the death of Lincoln.

Like his assassinated predecessor William McKinley, these mostly forgotten men felt no need to exercise the power and influence inherent in the presidency and were largely overshadowed by the congressional leaders, industrial tycoons and Wall Street bankers who dominated the American scene in the post-Civil War years. Roosevelt rejected their legacy and made the White House a powerful force in shaping the nation's political, economic and social life. The first president to begin his term in the 20th century, he had an impact on the governing of the country that endured long after he left office in 1909 after completing his second administration.

Theodore Rex--Morris recalls that Henry James used the phrase--is a lucid, insightful and sympathetic portrait of Roosevelt's seven and a half years as president. It follows the able biographer's Pulitzer Prize-winning The Rise of Theodore Roosevelt (1979) in which Morris chronicled Roosevelt's meteoric progress from his childhood in a prominent patrician family in Manhattan, through his years as a privileged Harvard preppy and tough North Dakota ranch hand, to a succession of political and military positions--among them the New York State assemblyman, New York City commissioner of police, heroic and famed leader of the Rough Riders in the Spanish-American War, a governor of New York, and, briefly, vice president.

Theodore Rex picks up Roosevelt's story with McKinley's death, when as Senator Mark Hanna famously declared, "that damned cowboy is the president of the United States." Roosevelt was then not quite 43. What follows is an almost play-by-play account of Roosevelt's years in power that begins with his breakneck dash by horseback and special trainride from his cabin in the wilderness of the Adirondack Mountains in upstate New York to Buffalo, where McKinley lay dead, the victim of an anarchist's bullet. In the ensuing 729 pages of narrative and endnotes, Morris focuses on the main events of Roosevelt's two administrations, the time of the Progressive Era in domestic politics and of America's arrival at the high table of world affairs.

These were event-filled years. Morris describes with drama and detail Roosevelt's success in resolving the crippling strike of Pennsylvania anthracite miners, an unprecedented intercession of the federal government in labor-management relations, and his drive to curb the power of the railroads and bust corporate trusts. He devotes long sections to Roosevelt's efforts to enact laws providing for what would nowadays be called consumer protection and his leadership of the conservation movement, which created the vast system of national parks and forests we know today.

Turning to the international scene, Morris discusses with precision and insight the machinations that led to the building of the Panama Canal, and his section on Roosevelt's intervention with the Japanese and Russians to end their 1904-05 war is particularly good. The president's efforts led to a peace treaty and won him a Nobel Peace Prize.

Morris brightens his descriptions and assessments of Roosevelt's official activities with colorful portraits of many of the president's contemporaries, from Elihu Root to William Howard Taft. But all of these characters are over-shadowed in Morris's pages by Roosevelt himself. A prodigious writer, voracious reader, tireless speaker, endless talker, keen hunter, wily politician, stern moralist, and loving paterfamilias, the charismatic president was a dominating figure devoted to an incredibly strenuous physical and intellectual life.

Foreign envoys clambered to keep up with him as he raced on horseback through Rock Creek Park and swam in the Potomac. Visiting Germans were astounded by his familiarity with the masterpieces of their fatherland's literature and his comprehensive knowledge of its affairs. He similarly impressed a delegation of Jews when he recited to them, impromptu, lines from Longfellow's poem, "The Jewish Cemetery at Newport."

Asked by his friend, Dr. Nicholas Murray Butler, the president of Columbia University, to recommend literary works his students should read, Roosevelt produced a long list that included histories, biographies, novels, poems, plays, and philosophical discourses written by Greek, Roman, English, French, Russian, Italian, German, and American authors, many of which he had read in their original languages. The newspapers gave him enormous coverage, treating the public to accounts of his bear hunting in Colorado and Mississippi, his boxing and wrestling prowess, his energetic romps with his five children at the White House and his family home on Long Island. These activities were in sharp contrast to the placid, sedentary style that McKinley and most of his recent predecessors had preferred.

Morris seamlessly mixes his accounts and assessments of Roosevelt's public activities with anecdotes and analyses--both his own and contemporaries'--of the private presidential persona. He writes in a lively style, but too often gets caught up in purple prose. He favors minutely detailed descriptions of places and events and relies too heavily for them on a few contemporary press stories. On several occasions he strays into psychohistory, telling us what Roosevelt was thinking at the time. These techniques, which best-selling biographers like Morris increasingly seem to favor, are distracting and make Theodore Rex considerably longer than it ought to be.

Morris is obviously an admirer of Roosevelt but he is objective in his assessments and is not overwhelmed by the long catalog of Roosevelt's achievements he records. He spells out how basically cautious the president often was in expounding changes in political and social policy and how willing he was to compromise with hostile conservatives in his own Republican party when he judged that necessary. Roosevelt discovered that he could make the White House a "bully pulpit." Morris finds, however, that the president was careful not to use it to try to move the public too quickly.

When Roosevelt called for reforms, he did so with wily and careful regard to the views and prejudices of his countrymen. He was eager to improve the lot of poor white citizens, as the "Square Deal" legislation he sponsored demonstrated, but failed to overcome congressional opposition to some of the changes he sought. And although he advocated a better life for African-Americans, he quickly retreated when his White House dinner invitation to black leader Booker T. Washington caused a political uproar early in his first term.

Playing games with history can be dangerous. But one can profitably ask how different America might have been if Leon Czolgosz had not gunned down William McKinley at the Pan-American Exhibition in 1901 and Theodore Roosevelt had remained trapped in the powerless position of vice president to which orthodox Republican leaders had deliberately consigned him. Had McKinley lived out his second term, these leaders would, in all likelihood, have successfully blocked Roosevelt's bid for the White House in 1904 and named a candidate more to their conservative tastes.

Under those circumstances, there very likely would have been a much less effective Progressive Era and little change in the way America was governed. Political and economic reform would eventually have come, but it would probably have taken a more radical and violent form spearheaded by politicians responding to the increasing animosity of the depressed poor for the prospering classes. If this projection is correct, Roosevelt was arguably one of the few presidents who made a major difference in American life and politics. Had he not been in the White House from 1901 to 1909, the United States would have been a very different place, not only in the decade when he led it, but for many years afterwards.

HOWARD B. SCHAFFER, author of Chester Bowles: New Dealer in the Cold War., now at Georgetown University, served as U.S. ambassador to Bangladesh.