Legend has it that after signing the Civil Rights Act of 1964 outlawing racial discrimination, President Lyndon Johnson grimly predicted his fellow Democrats would pay dearly for outlawing injustice. "We have lost the South for a generation," he purportedly said.

In her fascinating new book, "The Black Cabinet: The Untold Story of African Americans and Politics During the Age of Roosevelt," Jill Watts dissects the obverse of Johnson's lament: Why African-Americans, a once-loyal Republican constituency, fled the Party of Lincoln, the Great Emancipator, and embraced Democrats in the first place.

Answers are sprinkled throughout Watts' 449-page tome, revealing the hidden figures of a "brain trust" that lobbied, hectored and strong-armed President Franklin Roosevelt to cut African-Americans in on the New Deal. In exchange, the group guided by legendary educator Mary McLeod Bethune spurred a politically risky exodus of black voters from the GOP to FDR's Democrats, helping him win four terms.

Before then, Watts writes, Northern Republican presidents rewarded African-American electoral support with low-level federal patronage jobs (and benign indifference toward racism). But Southern segregationists dominated the Democratic Party, seemingly unperturbed by the Klan, Jim Crow laws and black sharecroppers starving on white-owned land.

A prototypical Black Cabinet of bureaucrats and policy wonks formed in the late 1800s; it pushed President William McKinley on race, but its fortunes turned on McKinley's assassination, Watts writes. Theodore Roosevelt, his successor, was ambivalent on race, but when word leaked that he'd secretly dined at the White House with Booker T. Washington, the fearless Roughrider slammed the back channel shut.

Decades later, amid the Great Depression, a conversation between an African-American manicurist and her well-connected white client led to a deal delivering the black vote to FDR, clinching the 1932 election, writes Watts. The Black Cabinet was soon resurrected under the dynamic Bethune, who found a BFF — and a powerful ally — in First Lady Eleanor Roosevelt.

At a postelection meeting, mostly male Cabinet members were buoyed by "a renewed spirit of determination and commitment," Watts writes. "And it was a woman who took the reins and began driving the men toward securing a voice for black people within the federal government."

But the convenience marriage between FDR and the Black Cabinet was rocky at best.

With one eye on powerful Dixiecrats in Congress, FDR chafed at Cabinet demands to integrate whites-only New Deal programs or deliver aid to destitute black communities. The Cabinet simultaneously fought for face time with the president and against deep-seated racism within the government they served.

The relentless Bethune won major uphill battles, but FDR wouldn't budge on bigger asks, including outlawing lynching, even as he scooped up black votes on Election Day.

Meticulously researched and elegantly written, "The Black Cabinet" is sprawling and epic, and Watts deftly re-creates whole scenes from archival material. With six main Cabinet characters, several subplots, infighting and at least three presidencies involved, however, it's a lot to take in.

The book clearly revolves around the larger-than-life Bethune, offering an object lesson on race and political expediency: A president who needs black votes but fears white backlash; black activists forging a pragmatic if unreliable allegiance with him; half-measures considered major victories. If you're starving, a crust of bread — stale, maybe a little moldy — is better than nothing.

Joseph P. Williams is an editor and reporter at U.S. News & World Report and a former assistant managing editor at the Star Tribune.

The Black Cabinet
By: Jill Watts.
Publisher: Grove Atlantic, 560 pages, $30.