Theodore H. White’s “The Making of the President 1960” revolutionized political journalism, and each presidential campaign since has brought one or more very solid chronicles – at first from White, more recently from imitators. Charles Peters, the longtime editor of the iconoclastic Washington Monthly, has gone these journalists one better. He has, as the social scientists say, “backcast” the White approach; his delightful new book, “Five Days in Philadelphia: The Amazing ‘We Want Willkie!’ Convention of 1940 and How It Freed FDR To Save the Western World” (PublicAffairs, 274 pages, $26) might have been called “The Making of the President 1940.” Mr. Peters’s book has another twist as well: Its central character is not the winner of the election, President Franklin D. Roosevelt, but rather the loser, Republican nominee Wendell Willkie.

Philip Roth trod the same ground in his recent novel, “The Plot Against America”; there, the Republicans nominate isolationist and Nazi sympathizer Charles Lindbergh, who goes on to deny FDR a third term and bring a sort of fascism to America. The present author argues that, but for Willkie’s astonishing rise and long-shot nomination, Great Britain might have lost the war even before Roosevelt’s second term concluded in January 1941. And the boundaries of fact and fiction nearly blur when we recall that the foreign policy plank of the 1940 Republican platform was drawn largely from an isolationist newspaper advertisement drafted by a covert Nazi agent working out of the office of New York Congressman Hamilton Fish.

Willkie’s principal opponents were two men who had not yet achieved the stations for which they are remembered. The Republicans’ front-runner during the 18 months before their convention in Philadelphia was Thomas E. Dewey, now best known as the New York governor who lost the 1944 election to FDR and then was upset by Harry Truman in 1948. Remarkably, in 1940, Dewey led the field from the modest perch of Manhattan district attorney.

Dewey’s twin claims to fame were as a crime-busting prosecutor and as the man who had run close behind Herbert Lehman in the 1938 contest for governor in New York. Such was the state of the Republican Party after Roosevelt’s landslide victories over Herbert Hoover in 1932 and Alf Landon in 1936 that this was considered enough for a presidential resume. In May 1940 a Gallup poll showed Dewey to be the choice of 67% of Republicans.

Next came Senator Robert Taft, an isolationist and son of the respected former president and chief justice. Taft is best known now for his leadership of Republican conservatives in the postwar period and for his battle with Dwight Eisenhower for the Republican nomination in 1952. He was the man who would place second to Willkie in the balloting in Philadelphia.

Willkie himself, a utility executive of modest Indiana roots, internationalist leanings, and New York sophistication (literary mistress and all), was a candidate invented by the Republican establishment. He was touted for the presidency by Henry Luce’s Fortune magazine as early as 1937. The April 1940 issue of the magazine was almost entirely devoted to boosting his chances and presenting his views; in May, 16 pages of Luce’s Life were also devoted to the cause.

Within two weeks of the Fortune spread, Willkie received 2,000 speaking invitations, but that only took him to 3% in Gallup’s May poll. Fortune editor Russell Davenport resigned to help run the Willkie campaign, enlisting advertising firms BBD&O and Young & Rubicam to help. Volunteers were led by a young Davis Polk attorney named Oren Root, who was protected within his firm by Thomas Lamont, the head of the House of Morgan. Not exactly a grassroots movement.

Yet these efforts did generate a swell in Willkie’s popularity at just the right moment, as the convention was opening. World events helped as well. Willkie’s 3% standing in the polls was registered two days before the Nazi invasion of the Low Countries. Six weeks later he had climbed to 29% versus Dewey’s 47%. Then France fell, and within six days Gallup recorded a swing of 15% toward Willkie and 18% away from the inexperienced Dewey. The day after the French surrender, the Republican Convention opened.

Mr. Peters’s lively narrative and eye for detail excel in the title scenes, the Convention’s five days and six ballots. We see Willkie’s superior organization as well as his superior luck. The chairman of the Committee on Arrangements dies suddenly, leaving a Willkie man at the helm. Willkie partisans are given tickets to pack the galleries, and Herbert Hoover’s rousing attack on Roosevelt – an attempt to stampede the convention in favor of his own nomination – is sabotaged by a faulty sound system. Hoover and Landon both oppose Willkie but dislike each other even more. Senator Arthur Vandenberg cannot seem to rouse himself to energetically pursue his own candidacy, perhaps enervated by the interventions of his mistress, a British spy.

Did Willkie’s nomination save freedom and the West? Mr. Peters cites no less an authority than Walter Lippmann in the affirmative. Still, the argument is less than convincing. Facing Willkie certainly made it easier for Roosevelt to pursue his 1940 initiatives of instituting the draft and arranging the destroyers-for-bases deal with the British. Yet even by campaign’s end, with the presidency dangling before him, Willkie said of the prospect of sending American boys to fight in Europe: “If you elect me president they will not be sent … if you reelect the third-term candidate, I believe they will be sent.” (To be sure, Roosevelt also promised that “your boys are not going to be sent into any foreign wars.”)

Willkie’s greatest service, which Mr. Peters charts, actually came after his 1940 defeat. His support for Roosevelt was crucial in obtaining approval for Roosevelt and Winston Churchill’s Lend-Lease agreement and persuading the House of Representatives – by one vote – to extend the draft in August 1941. These concluding chapters are less gripping than the account of the five days in Philadelphia, but on the whole Mr. Peters has produced an engaging volume in the very best Teddy White tradition.

Mr. Tofel is president of the International Freedom Center and author of the forthcoming “Sounding the Trumpet: The Making of John F. Kennedy’s Inaugural Address” (Ivan R. Dee).