K. Sabeel Rahman is an assistant professor of law at Brooklyn Law School, and a fellow at the Roosevelt Institute. He is the author of "Democracy Against Domination."

Every week, Americans are confronted with a growing sense of political and economic crisis, from the inequalities of the new economy to renewed debates about racial and gender discrimination to fresh stresses on our political institutions. During periods of crisis, the Constitution can provide a backstop, spelling out foundational rules and rights that offer a bulwark against the vicissitudes of politics. But the meaning of the Constitution can change precisely during these moments of pressure and flux. In his timely new book, "The Heart of the Constitution," Gerard N. Magliocca highlights how a key component of our Constitution, the Bill of Rights, has been a central touchstone for Americans throughout history, especially when faced with existential challenges to the legitimacy of American government.

Today, we colloquially see the Bill of Rights as encompassing that first batch of 1791 amendments to the Constitution, running from the First Amendment's freedoms of speech and association to the 10th Amendment reservation of powers to state governments. But these aren't the only rights spelled out in the Constitution. We tend to understand these rights as judicially enforceable limits on governmental action. But of course the Constitution articulates many other rights limiting government and, in some cases, implying affirmative obligations on the part of government. Article I protects individual liberties, such as the right to appeal one's detention through writs of habeas corpus, through explicit limits on congressional power. And later amendments, crucially, achieved the abolition of slavery (through the 13th Amendment); the expansion of equal protection, due process, and the privileges and immunities of citizenship (14th); and prohibitions against restrictions on voting rights on the basis of race (15th) or sex (19th), or through poll taxes (24th).

So why are these rights not considered part of the core Bill of Rights? Magliocca argues that the very idea of a bill of rights is a political and social construction. Our understandings of the existence of a bill of rights, which rights are part of the bill and what those rights mean have changed over time in response to different threats to the legitimacy and viability of American democracy. "The Bill of Rights," Magliocca writes, "is a mirror for how America sees itself," taking a different form "every political season."

Magliocca begins his story in the pre-founding era, looking at the early origins of a bill of rights. From the 1689 English Declaration of Rights to the Declaration of Independence to the Virginia Declaration of Rights in 1776, these early bills were soaring, but vague, articulations of general principles of democracy, rights and liberty, directed at legislatures and often coming as a preface to major legal documents, rather than as amendments following the end. They were suggestive and prefatory, not seen as specific, enforceable rights. James Madison himself, the author of those 1791 amendments, did not refer to them as a "bill of rights" at all.

The idea that these amendments constituted a specific Bill of Rights central to the meaning of the Constitution emerged much later. During the Civil War and Reconstruction, the framers of the 14th Amendment invoked the first 10 amendments as a bill of rights that, through the 14th Amendment, needed to be explicitly and directly applied to the states, in response to the experience of state-defended slavery and state suppression of anti-slavery advocacy. During the New Deal, Franklin Roosevelt invoked the Bill of Rights to help legitimize his wide-ranging efforts to alleviate the Great Depression; so long as the first 10 amendments were respected, FDR argued, critics of the New Deal were mistaken to view it as a threat to liberty. The Bill of Rights, in this view, became a touchstone for political legitimacy, effectively enabling new forms of expansive state action.

The Bill of Rights took on even greater significance with the start of World War II, as Roosevelt made the amendments central to the war effort, the key distinguishing factor between the totalitarianism of Hitler's Germany and the morality of American democracy. Opponents of the war and governmental action similarly invoked the Bill as an argument against unchecked government aggression. The Bill of Rights thus emerged through political appeals aimed at justifying new forms of state action — at home and abroad — and to bolster the legitimacy of American democracy in the face of domestic and global controversy.

By the mid-20th century, the Bill of Rights had become enshrined in our public imagination and in our political and legal consciousness. The Supreme Court increasingly referenced the 1791 amendments as a Bill of Rights, at times interpreting them as a set and viewing them with a special solicitude. This surprising ascent of the Bill in popular discourse is exemplified by Magliocca's retracing of how Americans treated the documents themselves. In a telling pair of anecdotes, Magliocca describes how a Union soldier arriving with Gen. William Sherman's troops in North Carolina in 1865 found a copy of the original amendments to the Constitution, then sold it for $5 to an Ohio businessman, who displayed it on his office wall for many years. Only in 1938 did the federal government transfer its official copy from its unceremonious storage spot in the basement of the State Department to the National Archives.

But this story of the elevation of the Bill of Rights to its central position in constitutional and political consciousness is also a story of the shifting meaning of the Bill itself. Indeed, the political purposes of the Bill of Rights over time help explain why our constitutional and political discourse today seems ill-equipped to handle the three central crises now facing American politics: the crisis of perpetual war and the post-9/11 surveillance state, the crisis of economic inequality, and the crises of systemic racial and gender discrimination and exclusion.

First, Magliocca highlights the role of the Bill of Rights in distinguishing America during the 20th century from the totalitarianism of Hitler's Nazi regime and Stalin's Soviet Union. But this also meant that the existence of the Bill of Rights was used to sanction expansive U.S. military intervention around the world. In a particularly telling and troubling episode, Magliocca recounts how Theodore Roosevelt invoked the Bill of Rights to legitimize American imperial control of the Philippines, instructing Philippine Gov. William Howard Taft to "respect" the first 10 amendments as a way to bolster the legitimacy of U.S. rule.

Second, the Cold War context of the elevation of the Bill of Rights meant that it was the civil liberties of the 1791 amendments — and not FDR's "Second Bill of Rights" for socioeconomic necessities — that got pride of place. The idea of a second bill faded away in the reaction against communism, as the Supreme Court during the 1970s systematically turned away from appeals to find implicit socioeconomic rights in the Constitution, often suggesting that these economic rights were not of the same stature or importance as the core Bill of Rights.

Third, the elevation of the Bill of Rights over the past century and a half has consistently effaced efforts to achieve a more inclusive and egalitarian view of American society. The 13th, 14th and 15th Amendments together espouse a radical commitment to establishing racial, economic, social and political equality, specifically aimed at uprooting the legacy of slavery. The 19th Amendment secured the vote for women, following decades of effort by feminist activists. These amendments were well-established parts of the Constitution when FDR and Harry Truman invoked the Bill in their propaganda efforts against Hitler and Stalin.

Indeed, the punchline of Magliocca's book is that our modern view of the Bill of Rights is far too stultifying. Giving pride of place to the 1791 amendments means the Bill is "locked in a gilded cage." Yet history shows that it was precisely the fluidity of the concept of a bill of rights that enabled it to serve as a vessel for debates over American values and identity; as the country faced new challenges both foreign and domestic, appeals to (and different definitions of) a foundational bill of rights enabled essential debates about inclusion, citizenship, war and the role of the state. Furthermore, especially before the 1940s, these debates were directed not just at the courts but at the American public more broadly.

Now, as we face a new set of crises, from war to inequality to structural exclusion, a more dynamic debate over a 21st-century bill of rights might offer some avenues forward. Magliocca's book can help us start that debate.

The Heart of the Constitution

How the Bill of Rights Became the Bill of Rights

By Gerard N. Magliocca

Oxford. 235 pp. $29.95