Some inequality of income and wealth is inevitable, if not necessary. If an economy is to function well, people need incentives to work hard and innovate.

The pertinent question is not whether income and wealth inequality is good or bad. It is at what point do these inequalities become so great as to pose a serious threat to our economy, our ideal of equal opportunity and our democracy.

We are near or have already reached that tipping point. As French economist Thomas Piketty shows beyond doubt in his “Capital in the Twenty-First Century,” we are heading back to levels of inequality not seen since the Gilded Age of the late 19th century. The dysfunctions of our economy and politics are not self-correcting when it comes to inequality.

But a return to the Gilded Age is not inevitable. It is incumbent on us to dedicate ourselves to reversing this diabolical trend. But in order to reform the system, we need a political movement for shared prosperity.

Herewith a short summary of what has happened, how it threatens the foundations of our society, why it has happened, and what we must do to reverse it.

What Has Happened

The data on widening inequality are remarkably and disturbingly clear. The Congressional Budget Office has found that between 1979 and 2007, the onset of the Great Recession, the gap in income—after federal taxes and transfer payments—more than tripled between the top 1 percent of the population and everyone else. The after-tax, after-transfer income of the top 1 percent increased by 275 percent, while it increased less than 40 percent for the middle three quintiles of the population and only 18 percent for the bottom quintile.

The gap has continued to widen in the recovery. According to the Census Bureau, median family and median household incomes have been falling, adjusted for inflation; while according to the data gathered by my colleague Emmanuel Saez, the income of the wealthiest 1 percent has soared by 31 percent. In fact, Saez has calculated that 95 percent of all economic gains since the recovery began have gone to the top 1 percent.

Wealth has become even more concentrated than income. An April 2013 Pew Research Center report found that from 2009 to 2011, “the mean net worth of households in the upper 7 percent of wealth distribution rose by an estimated 28 percent, while the mean net worth of households in the lower 93 percent dropped by 4 percent.”

Why It Threatens Our Society

This trend is now threatening the three foundation stones of our society: our economy, our ideal of equal opportunity and our democracy.

The economy. In the United States, consumer spending accounts for approximately 70 percent of economic activity. If consumers don’t have adequate purchasing power, businesses have no incentive to expand or hire additional workers. Because the rich spend a smaller proportion of their incomes than the middle class and the poor, it stands to reason that as a larger and larger share of the nation’s total income goes to the top, consumer demand is dampened. If the middle class is forced to borrow in order to maintain its standard of living, that dampening may come suddenly—when debt bubbles burst.

Consider that the two peak years of inequality over the past century—when the top 1 percent garnered more than 23 percent of total income—were 1928 and 2007. Each of these periods was preceded by substantial increases in borrowing, which ended notoriously in the Great Crash of 1929 and the near-meltdown of 2008.

The anemic recovery we are now experiencing is directly related to the decline in median household incomes after 2009, coupled with the inability or unwillingness of consumers to take on additional debt and of banks to finance that debt—wisely, given the damage wrought by the bursting debt bubble. We cannot have a growing economy without a growing and buoyant middle class. We cannot have a growing middle class if almost all of the economic gains go to the top 1 percent.

Equal opportunity. Widening inequality also challenges the nation’s core ideal of equal opportunity, because it hampers upward mobility. High inequality correlates with low upward mobility. Studies are not conclusive because the speed of upward mobility is difficult to measure.

But even under the unrealistic assumption that its velocity is no different today than it was thirty years ago—that someone born into a poor or lower-middle-class family today can move upward at the same rate as three decades ago—widening inequality still hampers upward mobility. That’s simply because the ladder is far longer now. The distance between its bottom and top rungs, and between every rung along the way, is far greater. Anyone ascending it at the same speed as before will necessarily make less progress upward.

In addition, when the middle class is in decline and median household incomes are dropping, there are fewer possibilities for upward mobility. A stressed middle class is also less willing to share the ladder of opportunity with those below it. For this reason, the issue of widening inequality cannot be separated from the problems of poverty and diminishing opportunities for those near the bottom. They are one and the same.

Democracy. The connection between widening inequality and the undermining of democracy has long been understood. As former Supreme Court Justice Louis Brandeis is famously alleged to have said in the early years of the last century, an era when robber barons dumped sacks of money on legislators’ desks, “We may have a democracy, or we may have great wealth concentrated in the hands of a few, but we cannot have both.”

As income and wealth flow upward, political power follows. Money flowing to political campaigns, lobbyists, think tanks, “expert” witnesses and media campaigns buys disproportionate influence. With all that money, no legislative bulwark can be high enough or strong enough to protect the democratic process.

The threat to our democracy also comes from the polarization that accompanies high levels of inequality. Partisanship—measured by some political scientists as the distance between median Republican and Democratic roll-call votes on key economic issues—almost directly tracks with the level of inequality. It reached high levels in the first decades of the twentieth century when inequality soared, and has reached similar levels in recent years.

When large numbers of Americans are working harder than ever but getting nowhere, and see most of the economic gains going to a small group at the top, they suspect the game is rigged. Some of these people can be persuaded that the culprit is big government; others, that the blame falls on the wealthy and big corporations. The result is fierce partisanship, fueled by anti-establishment populism on both the right and the left of the political spectrum.

Why It Has Happened

Between the end of World War II and the early 1970s, the median wage grew in tandem with productivity. Both roughly doubled in those years, adjusted for inflation. But after the 1970s, productivity continued to rise at roughly the same pace as before, while wages began to flatten. In part, this was due to the twin forces of globalization and labor-replacing technologies that began to hit the American workforce like strong winds—accelerating into massive storms in the 1980s and ’90s, and hurricanes since then.

Containers, satellite communication technologies, and cargo ships and planes radically reduced the cost of producing goods anywhere around the globe, thereby eliminating many manufacturing jobs or putting downward pressure on other wages. Automation, followed by computers, software, robotics, computer-controlled machine tools and widespread digitization, further eroded jobs and wages. These forces simultaneously undermined organized labor. Unionized companies faced increasing competitive pressures to outsource, automate or move to nonunion states.

These forces didn’t erode all incomes, however. In fact, they added to the value of complex work done by those who were well educated, well connected and fortunate enough to have chosen the right professions. Those lucky few who were perceived to be the most valuable saw their pay skyrocket.

But that’s only part of the story. Instead of responding to these gale-force winds with policies designed to upgrade the skills of Americans, modernize our infrastructure, strengthen our safety net and adapt the workforce—and pay for much of this with higher taxes on the wealthy—we did the reverse. We began disinvesting in education, job training and infrastructure. We began shredding our safety net. We made it harder for many Americans to join unions. (The decline in unionization directly correlates with the decline of the portion of income going to the middle class.) And we reduced taxes on the wealthy.

We also deregulated. Financial deregulation in particular made finance the most lucrative industry in America, as it had been in the 1920s. Here again, the parallels between the 1920s and recent years are striking, reflecting the same pattern of inequality.

Other advanced economies have faced the same gale-force winds but have not suffered the same inequalities as we have because they have helped their workforces adapt to the new economic realities—leaving the United States the most unequal of all advanced nations by far.

What We Must Do

There is no single solution for reversing widening inequality. Thomas Piketty’s monumental book “Capital in the Twenty-First Century” paints a troubling picture of societies dominated by a comparative few, whose cumulative wealth and unearned income overshadow the majority who rely on jobs and earned income. But our future is not set in stone, and Piketty’s description of past and current trends need not determine our path in the future. Here are ten initiatives that could reverse the trends described above:

1) Make work pay. The fastest-growing categories of work are retail, restaurant (including fast food), hospital (especially orderlies and staff), hotel, childcare and eldercare. But these jobs tend to pay very little. A first step toward making work pay is to raise the federal minimum wage to $15 an hour, pegging it to inflation; abolish the tipped minimum wage; and expand the Earned Income Tax Credit. No American who works full time should be in poverty.

2) Unionize low-wage workers. The rise and fall of the American middle class correlates almost exactly with the rise and fall of private-sector unions, because unions gave the middle class the bargaining power it needed to secure a fair share of the gains from economic growth. We need to reinvigorate unions, beginning with low-wage service occupations that are sheltered from global competition and from labor-replacing technologies. Lower-wage Americans deserve more bargaining power.

3) Invest in education. This investment should extend from early childhood through world-class primary and secondary schools, affordable public higher education, good technical education and lifelong learning. Education should not be thought of as a private investment; it is a public good that helps both individuals and the economy. Yet for too many Americans, high-quality education is unaffordable and unattainable. Every American should have an equal opportunity to make the most of herself or himself. High-quality education should be freely available to all, starting at the age of 3 and extending through four years of university or technical education.

4) Invest in infrastructure. Many working Americans—especially those on the lower rungs of the income ladder—are hobbled by an obsolete infrastructure that generates long commutes to work, excessively high home and rental prices, inadequate Internet access, insufficient power and water sources, and unnecessary environmental degradation. Every American should have access to an infrastructure suitable to the richest nation in the world.

5) Pay for these investments with higher taxes on the wealthy. Between the end of World War II and 1981 (when the wealthiest were getting paid a far lower share of total national income), the highest marginal federal income tax rate never fell below 70 percent, and the effective rate (including tax deductions and credits) hovered around 50 percent. But with Ronald Reagan’s tax cut of 1981, followed by George W. Bush’s tax cuts of 2001 and 2003, the taxes on top incomes were slashed, and tax loopholes favoring the wealthy were widened. The implicit promise—sometimes made explicit—was that the benefits from such cuts would trickle down to the broad middle class and even to the poor. As I’ve shown, however, nothing trickled down. At a time in American history when the after-tax incomes of the wealthy continue to soar, while median household incomes are falling, and when we must invest far more in education and infrastructure, it seems appropriate to raise the top marginal tax rate and close tax loopholes that disproportionately favor the wealthy.

6) Make the payroll tax progressive. Payroll taxes account for 40 percent of government revenues, yet they are not nearly as progressive as income taxes. One way to make the payroll tax more progressive would be to exempt the first $15,000 of wages and make up the difference by removing the cap on the portion of income subject to Social Security payroll taxes.

7) Raise the estate tax and eliminate the “stepped-up basis” for determining capital gains at death. As Piketty warns, the United States, like other rich nations, could be moving toward an oligarchy of inherited wealth and away from a meritocracy based on labor income. The most direct way to reduce the dominance of inherited wealth is to raise the estate tax by triggering it at $1 million of wealth per person rather than its current $5.34 million (and thereafter peg those levels to inflation). We should also eliminate the “stepped-up basis” rule that lets heirs avoid capital gains taxes on the appreciation of assets that occurred before the death of their benefactors.

8) Constrain Wall Street. The financial sector has added to the burdens of the middle class and the poor through excesses that were the proximate cause of an economic crisis in 2008, similar to the crisis of 1929. Even though capital requirements have been tightened and oversight strengthened, the biggest banks are still too big to fail, jail or curtail—and therefore capable of generating another crisis. The Glass-Steagall Act, which separated commercial- and investment-banking functions, should be resurrected in full, and the size of the nation’s biggest banks should be capped.

9) Give all Americans a share in future economic gains. The richest 10 percent of Americans own roughly 80 percent of the value of the nation’s capital stock; the richest 1 percent own about 35 percent. As the returns to capital continue to outpace the returns to labor, this allocation of ownership further aggravates inequality. Ownership should be broadened through a plan that would give every newborn American an “opportunity share” worth, say, $5,000 in a diversified index of stocks and bonds—which, compounded over time, would be worth considerably more. The share could be cashed in gradually starting at the age of 18.

10) Get big money out of politics. Last, but certainly not least, we must limit the political influence of the great accumulations of wealth that are threatening our democracy and drowning out the voices of average Americans. The Supreme Court’s 2010 Citizens United decision must be reversed—either by the Court itself, or by constitutional amendment. In the meantime, we must move toward the public financing of elections—for example, with the federal government giving presidential candidates, as well as House and Senate candidates in general elections, $2 for every $1 raised from small donors.

Building a Movement

It’s doubtful that these and other measures designed to reverse widening inequality will be enacted anytime soon. Having served in Washington, I know how difficult it is to get anything done unless the broad public understands what’s at stake and actively pushes for reform.

That’s why we need a movement for shared prosperity—a movement on a scale similar to the Progressive movement at the turn of the last century, which fueled the first progressive income tax and antitrust laws; the suffrage movement, which won women the vote; the labor movement, which helped animate the New Deal and fueled the great prosperity of the first three decades after World War II; the civil rights movement, which achieved the landmark Civil Rights and Voting Rights acts; and the environmental movement, which spawned the National Environmental Policy Act and other critical legislation.

Time and again, when the situation demands it, America has saved capitalism from its own excesses. We put ideology aside and do what’s necessary. No other nation is as fundamentally pragmatic. We will reverse the trend toward widening inequality eventually. We have no choice. But we must organize and mobilize in order that it be done.

[This essay appears in the current edition of “The Nation.”]