The niche genre of books exploring Abraham Lincoln’s individual speeches is nearly two decades old now, but remarkably robust, and still capable of fresh interpretations and eye-opening surprises. To call it a cottage industry for Lincoln historians would be an understatement, akin to calling Trump Tower a log cabin: This is one cottage that has built many mansions.

More than any other writer, it is Garry Wills who deserves credit for popularizing the specialty for the current generation. His “Lincoln at Gettysburg: The Words That Remade America” (1992) won a Pulitzer Prize and opened the floodgates. Among the fine historians who have subsequently made similar contributions are Ronald C. White Jr. (“Lincoln’s Greatest Speech: The Second Inaugural,” from 2002), Gabor Boritt (“The Gettysburg Gospel,” from 2006), and Allen C. Guelzo, who wrote the best book about the document that should have elicited a speech, “Lincoln’s Emancipation Proclamation” (2004). Fine recent works by both Mr. White (“The Eloquent President”) and Douglas L. Wilson (“Lincoln’s Sword”) further mined Lincoln’s evolving literary skills by analyzing a selection of his masterpieces. As a matter of full disclosure, my own 2004 volume, “Lincoln at Cooper Union,” took a stab at the same genre, arguing that the Illinois Republican’s debut New York oration did nothing less than elect him president.

What all these books have in common is that they examine speeches Lincoln delivered for the widely reported 1860 White House campaign, or the classics he subsequently produced as president. Lewis E. Lehrman’s innovative new book “Lincoln at Peoria” (Stackpole Books, 412 pages, $29.95) stands apart. It is the first to parse a speech from the 1850s, and in that regard alone it is an audacious undertaking. It challenges readers to explore an oration that antedates Lincoln’s sacred (and more familiar) efforts; that he delivered at the beginning, not the apex, of his national fame; that contains few golden sound bites that now live in Bartlett’s; that is numbingly long (over 12,000 words, requiring more than three hours of endurance from both orator and audience — Lincoln was a famously slow speaker); that contains sections that seem uncomfortably racist — certainly by modern standards; and, finally, that took place not in the media centers of Washington or New York (or the hallowed soil of Gettysburg, press corps in tow), but literally played in Peoria.

So why on earth should we care? Because, as Mr. Lehrman shows clearly and convincingly in this exhaustively researched and persuasively argued book, Lincoln’s 1854 Peoria speech poured the foundation — rhetorical and philosophical alike — that undergirded all of his later oratorical triumphs, and much of his presidential decision-making, including his history-altering pledge to preserve the Union and eradicate slavery.

It was at Peoria, Mr. Lehrman demonstrates, that Lincoln not only introduced a fresh new speaking style — breathtakingly simple, relentlessly analytical, and magnanimously free of the sarcasm so prevalent in his earlier orations — but also unveiled his new commitment to America’s founding doctrine: the “all men are created equal” guarantee of the Declaration of Independence. Lincoln’s revolutionary new style (“Euclidian” in its logic, “Socratic” in its dialectical approach, as Mr. Lehrman describes it), together with his new, equality-suffused “consistency of principle,” were alike destined to endure throbbingly in Lincoln’s subsequent speeches, the author shows, nourishing his anti-slavery dedication and defining his communication skills. Without Peoria, Mr. Lehrman argues, Lincoln not only could not have called for a new birth of freedom at Gettysburg or for malice toward none at his second inaugural. He might not have earned the presidency to begin with.

So why has Peoria eluded us? For one thing, Lincoln never saved the manuscript. We have no talismanic relic to cherish, only newspaper reprints later collected in books. For another, Peoria was but one (albeit the best) of several similar orations that Lincoln offered after being “aroused” back into politics — his own word — by the 1854 passage of the bitterly contested Congressional Kansas-Nebraska Act, whose author, Stephen A. Douglas, was Lincoln’s longtime political rival.

For the previous five years, Lincoln had found himself out of office after a frustrating single term in Congress. Back home, he took himself out of the game entirely, returning to his law practice and watching helplessly as his beloved Whig party began disintegrating. But Kansas-Nebraska changed everything. It repealed the 34-year-old Missouri Compromise, which had kept a tenuous lid on the smoldering slavery issue. More threateningly, Douglas’s controversial new legislation authorized residents of the nation’s new western territories — whether they lived above or below the old 363 latitude line that had long divided North from South — to vote for themselves on whether or not to introduce slavery.

To Lincoln, who believed that the founders had meant to prohibit such expansion in order to place slavery “in the course of ultimate extinction,” the new law represented a hideous violation of first principles; the Declaration of Independence was an “immortal emblem of humanity” which slavery violated, and slavery migration mocked. Lincoln had found his issue. And by pursuing it to the exclusion of all others, he transformed himself into what Mr. Lehrman calls “the self-tutored historian of the American Founding.”

The Peoria speech represented a major coming-out for this “new” Lincoln, but as Mr. Lehrman explains, its reputation has been plagued by further complications. The address was not actually a stand-alone speaking opportunity but a rejoinder from a little-known series of Lincoln-Douglas debates, which took place four years before the celebrated one-on-one face-offs that Lincoln and Douglas staged when they ran against each other for the U.S. Senate. Peoria was but one address in a long 1854 campaign that asked Illinois voters to line up on Kansas-Nebraska: an informal referendum on slavery expansion.

In practical terms, Lincoln also hoped that enough anti-Nebraska state legislators might be elected by that fall to choose him as a U.S. senator the following winter (in those days, state senators and assemblymen elected senators, not a direct vote of the people). Thus, Peoria was Lincoln’s chance for a trifecta: his dramatic re-entry into politics, his first major anti-slavery address, and his first debate against Douglas for the Senate. Though Illinois’ voters ultimately failed to denounce Kansas-Nebraska, and its legislators rejected Lincoln for the Senate (he came heartbreakingly close), the speech was far more than a “rehearsal” for the famous 1858 Lincoln-Douglas debates. At Peoria, Lincoln did nothing less, argues Mr. Lehrman, than “lay the intellectual groundwork for his presidential nomination,” introducing principles, he says, that “guided the Lincoln presidency.”

The Peoria text itself more than justifies Mr. Lehrman’s enthusiasm. Highlights are sprinkled throughout this book, and the address is reprinted in full — more than 54 pages — as an appendix. It bears careful reading. “The doctrine of self-government is right — absolutely and eternally right,” Lincoln says, in words as timeless as any he ever wrote. “When the white man governs himself, that is self-government; but when he governs himself and also governs another man, that is more than self-government — that is despotism. If the negro is a man, why then my ancient faith teaches me that ‘all men are created equal;’ and that there can be no moral right in connection with one man’s making a slave of another.” And then: “Allow ALL the governed an equal voice in the government, and that, and that only is self-government.”

Such words elevated the discourse on slavery even as they elevated Lincoln himself. But Mr. Lehrman does not shrink from frankly discussing the darker side of the Peoria speech as well, in which Lincoln, even as he espoused freedom of opportunity for all, shrank from the notion of racial equality for blacks (“[M]ake them politically and socially our equals? My own feelings will not admit of this; and if mine would, we well know that those of the great mass of white people will not”). Lincoln was a politician, not an apostle, and as such he recognized what historian LaWanda Cox once called “the limits of the possible” in the racist 1850s (supposedly enlightened Illinois had enacted laws to bar free blacks from its borders!). No western politician of the day could possibly espouse racial equality and hope for electoral success, and besides, Lincoln probably did believe in the notion of white superiority at the time.

But Mr. Lehrman will not fall into the trap of judging this quintessential 19th-century man by 21st-century standards. Firmly occupying the enlightened side of the political spectrum of the age, Lincoln emerges here as a man of his own time — though one who marshaled the backbone, ethics, and talent to tilt the balance toward freedom. At Peoria, Lincoln first told his state that there could be no union without liberty, a declaration that enabled him to tell the entire nation, seven years later, that there could be no liberty without union.

This is a complicated story indeed, but Mr. Lehrman makes it a page-turner, rendering it not only comprehensible, but also compelling: There is never a moment in this book when philosophy overwhelms politics, or politics suffocates personality. Authentic voices from the past recall the vanished culture of enthusiastic mass participation in politics, and these vivid period recollections are accompanied by reams of commentary from distinguished historians of many generations who have written previously on the subject. Though their observations remain valuable, none ever gave Peoria the singular focus that the current book does. It is a measure of his respect for his scholarly predecessors, and of his own talent for acute observation and analysis, that Lehrman generously gives his colleagues their full due while going them one better.

In “Lincoln at Cooper Union,” I argued that without his 1860 speech in New York, Lincoln might not have been elected to the White House. In “Lincoln at Peoria,” Lewis Lehrman convinces us that without that earlier, stylistically and philosophically formative speech in Illinois, it might not have mattered. Cooper Union might have made Lincoln president. But Peoria made Lincoln Lincoln.

Mr. Holzer is co-chairman of the United States Lincoln Bicentennial Commission and the author of more than two dozen books on the 16th president.