It used to be said of Arthur M. Schlesinger Jr., that “he writes as he votes.” The question then emerges whether his anointed successor, Princeton historian Sean Wilentz, could avoid the same pitfalls, particularly when writing about Ronald Reagan. After all, Mr. Wilentz is not only one of America’s pre-eminent historians, but a well-known Democratic activist, a fierce defender of the legacy of Bill Clinton’s presidency, a leader in the fight against his impeachment, and, today, a vocal supporter of the presidential campaign of Hillary Clinton.

In “The Age of Reagan” (HarperCollins, 564 pages, $27.95), Mr. Wilentz seeks to assure his readers that he has strived to lay aside his personal views, and to judge “the past scrupulously” by engaging in “a willing suspension of [his] own beliefs.” This reader is glad to report that, in this study of the Reagan presidency and its impact on America, Mr. Wilentz achieves this aim, for the most part — although at times, he does drift into the type of anti-Reagan attacks one heard during the 1980s from Reagan’s liberal opponents. Above all, what Mr. Wilentz seeks to do is rescue the real Reagan from what he calls the mythological president offered by supporters on the right and critics on the left. In so doing, he makes judgments that will rankle both.

What will particularly upset many partisan Democrats is Mr. Wilentz’s conclusion that Reagan stands with presidents such as Jackson, Roosevelt, and Lincoln as a leader “who for better or worse have put their political stamp indelibly on their time.” Moreover, he argues, Reagan deserves credit as a president who took ideas seriously, and more than his immediate predecessors, redefined the politics of the era, thus “reshaping the basic terms on which politics and government would be conducted long after he left office.” Mr. Wilentz acknowledges and praises Reagan’s optimistic spirit, and the way in which he energized the public and made Americans once again proud of their nation, lifting his countrymen out of the doldrums suffered during the Nixon, Ford, and Carter years.

Mr. Wilentz begins his book with a broad overview of the Ford and Carter presidencies. In those years, Democrats depended upon old and outdated bromides. Jimmy Carter won, he suggests, because he broke with the old liberalism. Indeed, he attributes Mr. Carter’s slim victory over Ford to his identification with the party’s Southern wing, his outsider status in relation to the Beltway crowd, and his being a politician who was clearly not part of the old Humphrey liberal wing. Yet Mr. Carter squandered his opportunity to lead to something new. Instead, he tried to update Wilsonian Southern conservatism, when he might have instead updated and revised liberalism in a way that would grip Americans.

Mr. Carter’s failures, Mr. Wilentz shows, coincided with the rise and growing strength of the conservative counter-establishment that had begun with the Goldwater campaign, and was now organizing a new Republican base that easily broke into Mr. Carter’s Southern and Western strongholds. In 1980, Mr. Carter and the Democratic party as a whole faced a new conservative movement, one that successfully united social conservatives, anti-communist activists, and advocates of fiscal responsibility and small government into one united camp.

The differences between Mr. Carter and Reagan could not have been clearer. “Carter spoke philosophically of ambiguities and limits, Reagan spoke with splendid simplicity about an unbounded American future.” Mr. Carter’s presidency was one of failure both at home and abroad while Reagan, Mr. Wilentz writes, “promised a bright new future.” Moreover, Reagan’s conservatism was brought to the public in words that purposefully emulated those spoken by Roosevelt, whom Reagan had himself supported in the 1940s. Acknowledging Reagan’s conservative agenda, Mr. Wilentz argues that Reagan cannot be defined by simple terms such as conservative, hawk, or pro-business. Reaganism was unique, he writes, “its own distinctive blend of dogma, pragmatism, and above all, mythology.” He started his presidency promising deep tax cuts, an increase in defense spending, and an end to the federal deficit. But most importantly, Reagan pointed toward an end to the postwar syndrome of defeat abroad.

Mr. Wilentz hardly excuses all of Reagan’s policies and actions. Indeed, his own breezy account of the Reagan years faults the president with wrongheadedness in both domestic and foreign affairs. Mr. Wilentz skewers supply-side economics and tax cuts, arguing that, the claims of conservatives to the contrary, they proved to be a failure. Yet he acknowledges that the greatest domestic achievement Reagan had was “the revival of the sputtering economy.” Mr. Wilentz notes that when he took office, the inflation rate was 12%, the prime lending rate had jumped to 20%, and unemployment was 7.2%. Eight years later, the respective figures were 4.4%, 9.3%, and 5.5%. It was what he calls “the longest continuous period of peacetime economic growth in the nation’s history.” America’s GDP increased twofold, and 18 million jobs were created. Clearly, Reagan was doing something right.

On foreign policy, Mr. Wilentz’s argument is simple. He praises Reagan for his turn toward pragmatism, symbolized by his reliance on foreign policy realists who were less ideological and willing to engage in frank negotiations with the Soviets, and to reach agreements that would scale down armaments. Reagan bucked the advice of hard-line conservatives and neocons, moving on his own to welcome Gorbachev as an adversary with whom he could do business and enter into negotiations. He notes that in 1988, conservatives who today claim Reagan as their own for his role in downing the “evil empire” at the time condemned him. Howard Phillips called Reagan “a useful idiot for Kremlin propaganda,” and the Washington Times compared him to Neville Chamberlain. When Reagan prepared to negotiate with Gorbachev in Geneva, Tom Bethell wrote that Reagan was “doing what the Soviets want.” George Will accused him of accelerating “the moral disarmament of the West,” and National Review argued that Gorbachev was ruling in the manner of a “vintage Stalin.” Others warned against trusting Gorbachev at all. The Heritage Foundation called him a Stalinist who “brought no essential change in the Soviet political scene.”

Conservatives will not agree with Mr. Wilentz’s argument that Reagan opposed the hard-line position of conservatives in his administration; liberals will not like the full credit Mr. Wilentz gives Reagan for his willingness to jump into negotiations with Mikhail Gorbachev, and for genuinely seeking to avoid nuclear war. Mr. Wilentz sees him as a strong and necessary president. Reagan’s “ability to dispense with dogma … and negotiate with Gorbachev helped bring an end to a nuclear arms race that had terrified the world for forty years.” Indeed, Mr. Wilentz goes so far as to call the end of the Cold War “one of the greatest achievements by any president of the United States — and arguably the great single presidential achievement since 1945.”

But Reagan also delivered a broader contribution, Mr. Wilentz argues. His final judgment is that Reagan was great because he understood American politics, and aside from Iran-Contra, he “practiced the art of compromise shrewdly.” He had more of an effect on the temper of the times and the shift of the nation to the right, thereby “reshaping the basic terms on which politics and government would be conducted long after he left office.”

Mr. Wilentz should have ended his book with that sentence. Instead, he writes many chapters on the Bush presidency and the Clinton years, even ending with the expected attack on George W. Bush, whom Mr. Wilentz elsewhere calls the worst president in American history.

Mr. Wilentz reaches judgments with which many people, including this writer, will disagree. He supports the Reagan whose policies he likes, and criticizes fiercely the Reagan whose policies he opposes. In those pages, he fails his own test of remaining objective. Yet despite this flaw, any student of our past will learn a great deal from his book about what Ronald Reagan did for America, and how he changed our nation. Mr. Wilentz has written an essential book for our times.

Mr. Radosh, adjunct fellow at the Hudson Institute, is author of “Divided They Fell: The Demise of the Democratic Party, 1964-1996,” among other books.