In this age of political posturing, the White House tries to scoop itself on the State of the Union message by pre-publicizing it in dribs and drabs. But in Lincoln’s day, leaking this affair of state was treated as a scandal—one that might have meant a world of hurt for Honest Abe just as his 11-year-old son died and the Civil War started to turn the Union's way.

Hours before Lincoln sent his first State of the Union message to Congress on December 3, 1861—in those days, the message was delivered in writing—an anti-Lincoln newspaper, the New York Herald, published a few excerpts. No big deal, right? Wrong. Lincoln scrambled to cover up the apparent source of the leak—his wife—and to save his administration from a public humiliation.

The House Judiciary Committee pursued the leaker, or leakers, with a vengeance. It already had reason to suspect their identities before its investigation began in February 1862. The correspondent for the Herald’s archrival New York Tribune had fingered, as a go-between, a flatterer and social climber named Henry Wikoff, who was getting “news from the White House … from women … members of the president’s family.”

The Chevalier Wikoff, as he liked to be known, for some obscure past service to the Spanish crown, had hobnobbed with royalty all over Europe. Now in Washington, he served as a secret correspondent for the Herald (he was friends with the newspaper’s founder and editor, James Gordon Bennett) and had exploited Mary Lincoln’s social insecurities to insinuate himself into her affections.

The involvement of Mrs. Lincoln may help explain the vigor of the committee’s pursuit. John Hickman, the severe-looking chairman, was an eloquently anti-slavery Republican from Pennsylvania who presumably shared many northerners’ scorn for the president’s wife. Besides having a brother, three half-brothers, and three brothers-in-law in the Confederate army, Mrs. Lincoln was assailed for lavishly redecorating the Executive Mansion while Union soldiers needed blankets. Earlier in February, the press had pilloried the First Lady (to whom that term was first applied, by the correspondent for The Times of London) for putting on an “ostentatious” and “most unseemly” White House ball....