This was the year that intergenerational social mobility broke out of the wonkosphere into mainstream political debate. Most of the thanks must go to President Obama, who has highlighted the challenge of stagnant social mobility in the U.S.—alongside income inequality and low pay.

Obama finished the year with a widely-publicized speech on inequality that addressed, head on, the problem of the transmission of both affluence and poverty between generations:

“The combined trends of increased inequality and decreasing mobility pose a fundamental threat to the American Dream, our way of life, and what we stand for around the globe. And it is not simply a moral claim that I’m making here. There are practical consequences to rising inequality and reduced mobility.”

The president even cited figures based on the movement of people between different quintiles of the income distribution. Oh, happy day for us number crunchers! (Actually he somewhat misstated the figures, but that’s another story.)

Not everyone on the Democrat side is delighted with the president’s new populism. Centrist Democrats, such as Charles Lane over at the Post, point out that just 17% of voters think tackling inequality is a top priority for the government. But tackling the low rates of social mobility is hardly a left-wing agenda, unless the American Dream has been a myth all along.

One question hinted at by the president, is the relationship between income inequality and intergenerational mobility. This has been a hot topic in scholarly circles ever since Alan Krueger, then Chairman of the Council of Economic Advisers, postulated a causal connection in his “Great Gatsby Curve”. In his speech, however, the president was careful to refer only to the “combined trends” of inequality and immobility, effectively sidestepping the causal question.

The extent of any causal connection is a scholarly side-issue. Inequality and immobility is a toxic combination in any case. An unequal, immobile society will mutate inescapably into a stratified, one, with sharply separated classes, perpetuated between generations. It is not the causal link between inequality and immobility that matters, rather the consequences of their co-existence.

There is a moral justification for a society with high inequality offset by high mobility, grounded in liberal ideas of freedom and fairness. And there is a moral justification for a society with low mobility, softened by low inequality, based on left-of-center egalitarian ideals. But there is no moral justification for a society with a large gap between rich and poor, and little movement between the two. That’s the toxic combination we’re suffering from right now.

When it comes to solutions, the president continues to lean heavily on traditional center-left idea: a higher minimum wage, more spending on education, especially pre-K, and higher taxes on the very wealthy. All well and good—but, as the president himself acknowledges, there are other factors at work too. Parental responsibility, family stability, aspirational community culture, individual character: all areas where Democrats start shifting uncomfortably in their seats.

Robert Putnam, the Harvard professor whose thinking is heavily influencing the president, points to the “red truths” about family, responsibility and parenting and the “blue truths” about job availability and educational quality. (Note: Putnam is working on a book that’s sure to become a defining text.)

Social mobility is, then, a necessarily bipartisan issue; you’d be hard pressed to find a Republican who would support the current rates of upward mobility in the U.S.

Obama threw a gauntlet across the aisle in his December speech:

“If Republicans have concrete plans that will actually reduce inequality, build the middle class, provide more ladders of opportunity to the poor, let’s hear them. I want to know what they are. If you don’t think we should raise the minimum wage, let’s hear your idea to increase people’s earnings. If you don’t think every child should have access to preschool, tell us what you’d do differently to give them a better shot.”

As it happens, Congressman Paul Ryan will be here at Brookings in January, setting out his agenda for improving mobility: in effect, answering the president’s challenge.

Let us hope that the greater attention being paid, at long last, to the issue of social mobility leads to a conversation across party lines, and one based on evidence rather than ideology. Optimistic, perhaps. But it’s the holidays. Surely a touch of optimism is permitted, at least today.