Rick Santorum is not exactly an odds-on favorite in the race for the Republican presidential nomination. And perhaps that's why it was Santorum who felt free to articulate an important truth in the GOP candidates' debate in Las Vegas on Tuesday night.

"Believe it or not, studies have been done that show that in Western Europe, people at the lower parts of the income scale actually have a better mobility going up the ladder now than in America. "

Santorum was referring to studies like this one, conducted at the Brookings Institution by a group led by Isabel Sawhill:

Men born into the poorest fifth of families in the United States in 1958 had a higher likelihood of ending up in the bottom fifth of the earnings distribution than did males similarly positioned in five Northern European countries — 42 percent in the United States, compared to 25 to 30 percent in the other countries. Furthermore, in the United States, only 8 percent make the "rags to riches" climb from bottom to top rung in one generation, while 11 to 14 percent do so in other countries.

(To view a helpful Brookings chart, click here, and see page 39, figure 2.)

The American dream is still alive. It's just more likely to come true in Denmark than in the USA. In fact, the American dream is less likely to come true in the USA than in any other major economy except the United Kingdom's.

The freezing of income mobility is distinct from, but probably related to, two other important trends in American life: The stagnation of middle-class incomes and the widening of the gap between rich and poor.

The American dream is less likely to come true in the USA than in any other major economy except the United Kingdom's.

A generation ago, an American family did not need to "climb the ladder" to become better off. If a family started in the dead middle of the income distribution in 1947 and ended in the dead middle of the distribution in 1973, it still saw its standard of living approximately double. By contrast, middle-class incomes barely budged in the quarter century leading up to 2007.

At the same time, the richest have pulled away from the middle — and the richest of the rich have pulled away from the merely affluent.

Conceptually, you could imagine a highly unequal society with rapid income mobility. You could imagine a society with little mobility, but in which all classes were getting richer at approximately the same pace. America, however, is a society of widening inequality, hardening class lines, and stagnating living standards for most people. And all of these trends rely on numbers from before the economic crisis and before the election of Barack Obama.

What (if anything) should be done? That's a great discussion to have. But even to start the discussion we have to acknowledge some uncongenial facts. One long-odds Republican is willing to do that. We'll know we're ready to act when some of the more highly ranked Republicans begin to follow where Rick Santorum has led.