A stratified society in which the bottom and top are mostly locked in place is not just morally offensive; it is unstable. Recessions are more frequent in such countries. A widely praised 2012 book, “Why Nations Fail,” argues that historically when the ruling elites have pulled up the ladder and kept newcomers from getting a foothold, their economies have suffocated and died. “The most pernicious fact of inequality is when it translates into political inequality,” said Daron Acemoglu, a co-author of the book and a Massachusetts Institute of Technology economist. “That means our democracy ceases to function because some people have so much money they command greater power.” The rich spend heavily on lobbyists and campaign donations to secure tax breaks and tariff advantages and bailouts that perpetuate their status. Not only does a dynamic economy stagnate, but the left-out citizenry becomes disillusioned and cynical. Sound familiar?

The left-left sees economic inequality as mainly a problem of distribution — the accumulation of vast wealth that never really trickles down from on high. Their prescription is to tax the 1 percent and close corporate loopholes, using the new revenues to subsidize the needs of the poor and middle class. They would string the safety net higher: expand Social Security, hold Medicare inviolate, extend unemployment insurance, protect food stamps, create more low-income housing. They would raise the minimum wage.

The center-left — and that includes President Obama, most of the time — sees the problem and the solutions as more complicated. Yes, you want to provide greater security for those without independent means (see Obamacare), but you also need to create opportunity, which means, first and foremost, jobs. Yes, you can raise taxes on the rich, but you don’t want to punish success. “You want to increase social mobility by providing an opportunity for the bottom to become rich, not forcing the rich to become poor,” said Acemoglu, who aligns more with the center than with the populists.

The center-left (I’m somewhat oversimplifying these categories) agrees on the menace of inequality, but places equal or greater emphasis on the fact that the economy is not growing the way it did for most of the last century. The sluggish growth means that not only are the poorest stuck at the bottom, but the broad middle is in economic decline. “The central economic problem is that a middle-class wage does not make a middle-class life anymore,” said Jon Cowan, president of Third Way. This diagnosis leads to somewhat different priorities. Of the arguments that pit Democrat against Democrat, three strike me as most important:

The first is how to restart the engine of growth. The populists favor putting more money in the hands of the bottom and middle, who will then spend us back to economic vigor. This is classic Keynesian thinking, largely vindicated by history. But the center-left points out that our economy is already 70 percent dependent on domestic consumers, way more than other developed countries. Liberals of all shades favor greater government investments in education, infrastructure, clean energy and other research. But they divide over policies that might unleash the energy of the private sector. In a line from his speech that was not widely quoted, President Obama said, “The fact is if you’re a progressive and you want to help the middle class and the working poor, you’ve still got to be concerned about competitiveness and productivity and business confidence that spurs private-sector investment.” While closing loopholes, Obama would also lower corporate tax rates; he would do trade deals to expand our diminishing share of foreign markets; he would shrink long-term deficits and streamline regulations.

The second argument is over entitlements. The left-left seems to believe that government investments — roads and bridges, clean energy, education, etc. — and more-generous safety-net benefits can all be had by milking the rich and cutting military spending. Most centrists would raise taxes some and cut defense spending some, but they say that unless we also curb the growth of entitlements, the stampede of baby boomers into Social Security and Medicare will crowd out everything else. Between now and 2030, the working-age population that pays into Social Security grows by 15 percent; the over-65 population that withdraws from Social Security grows by 65 percent. No one on the left favors entrusting Social Security and Medicare to the mercies of the private marketplace, as some Republicans do. But while the left tends to treat entitlements as sacred (Senator Warren and others would increase benefits for everyone, even the rich), centrists favor measures to slow the growth of entitlements: using a cost-of-living adjustment (COLA) formula that more accurately reflects how people spend, cutting benefits for those who don’t really need them, possibly extending the retirement age a couple of years, and using the government’s leverage to drive down the costs of medical care. The tension between entitlements and investment is a Third Way obsession. In a column and two blog posts last year (here, and here, and here) I sympathized with the case for imposing some restraint on entitlements. I still do.