H. W. Brands is the author of “Reagan: The Life” and other books of American history.

If Donald Trump wins the Republican nomination for president over the strenuous efforts of party elites to derail him, he ought to send a note of thanks to Geoffrey Cowan. Almost 50 years ago, Cowan led a campaign among the Democrats to strengthen the system of primary elections and reduce the power of party bosses. The campaign succeeded, giving the Democrats George McGovern in 1972 and spilling over into the GOP in time for Ronald Reagan to demonstrate in 1980 that primary voters were less worried about his age than the party pros.

Cowan is currently president of the Annenberg Foundation Trust and a professor at the University of Southern California. In “Let the People Rule,” he examines the origins of the primary system during the Progressive era of the early 20th century. Progressives decried the debilitation of democracy at the hands of corporate moguls and political bosses. They tackled the moguls with antitrust laws and business regulations; they circumvented the bosses with reforms that made democracy more direct. These included the popular election of senators, the initiative and the referendum, and primary elections.

In 1910 Oregon adopted a measure establishing the nation’s first presidential primaries. The idea had sufficient appeal that several other states scheduled primaries ahead of the 1912 presidential election. In that year Theodore Roosevelt, the former president, challenged William Howard Taft, the incumbent, for the Republican nomination. Taft controlled the party machinery, but Roosevelt hoped to leverage his personal popularity against the president.

Roosevelt had been tepid on the subject of primaries, but finding his way back to the White House blocked by the Taft regulars, he had a conversion experience. The primary, he suddenly proclaimed, was essential to good government. “I believe that the majority of the plain people of the United States will, day in and day out, make fewer mistakes in governing themselves than any smaller class or body of men, no matter what their training, will make in trying to govern them,” he told a packed house at Carnegie Hall in March 1912.

Cowan tells his story with great verve. He relates the experience of Roosevelt supporters in Oklahoma, who included veterans of TR’s Rough Rider regiment from the Spanish-American War. “Our fellows put up a great fight lasting all day and until four in the morning,” one of them wrote to Roosevelt. “One man dropped dead and two or three were carried out unconscious. The state chairman, Harris, a Taft man, was told if he tried to put over any crooked deals from the chair that he wouldn’t get out of the hall alive. Feelings ran so high that gun-play was expected. Indeed, I am told that one of Roosevelt’s men stood behind Harris with his hand on his gun ready for an emergency.”

Cowan’s tale is packed with such vignettes, portraying a pre-radio, pre-television, pre-Internet time when politics was conducted face to face. Cowan’s Roosevelt slashes the air with his right hand while making key points; he clips his words with a speaking style that one contemporary likened to biting off tenpenny nails. This Roosevelt was also capable of turning on a dime without conceding that he had gone anywhere but straight. “Let the people rule,” Roosevelt declared, but he didn’t mean all the people all the time. When it served his purposes to include African Americans, he was more than happy to accept their votes. But when the black vote worked against him — when, having won most of the primaries but lost the Republican nomination to Taft, he became the nominee of the Progressive Party and feared frightening off Southern whites — he took pains to exclude them. “I believe the great majority of the negroes in the South are wholly unfit for suffrage,” he declared in what was either a moment of candor or a moment of expedience. Cowan surmises it was a bit of both.

He nonetheless admires Roosevelt for opening presidential politics to greater participation. Cowan acknowledges the drawbacks of primaries. “The primary process has produced a new class of political leaders and insiders who are not necessarily representative of the general public or even of the party,” he writes. Primaries tempt or compel candidates to appease the most extreme elements in their parties. And they amplify the influence of money. “Primary campaigns have become so costly that candidates are forced to spend much of their time raising money and trying to win the hearts of a few very large donors.”

On balance, though, Cowan applauds what Roosevelt wrought. If not for primaries, he suggests, neither John Kennedy in 1960 nor Barack Obama in 2008 would have become president. And Reagan might have lost his third and doubtless final try to gain the White House. Cowan tips his cap to TR in concluding that primaries have indeed, as Roosevelt promised they would, “given the people the right to rule.”

And given them the right to make Trump the Republican nominee. Should such an outcome occur, those frustrated GOP regulars might wish TR had wrought less well.

Let the People Rule
Theodore Roosevelt and the Birth of the Presidential Primary

By Geoffrey Cowan

Norton. 404 pp. $27.95