He labels this group Fishtown, after the Philadelphia neighborhood that has been the home of low-income whites since it was first settled in the early 19th century.
In Fishtown, he reports, one-third of men age 30 to 49 are not making a living, one-fifth of women are single mothers raising children, and nearly 40 percent have no involvement in a secular or religious organization.
The result is that the children being raised in such settings have the odds heavily stacked against them. Santorum made this point vividly, and Mitt Romney chimed in his agreement.
These findings turn some conventional political wisdom on its head. They tend to contradict the liberals who blame increasing income disparity on free-market economics. In fact, it is driven in large part by personal behavior and choices.
They also undermine the conservatives who say that a liberation-minded upper class has been undermining traditional values to which more downscale Americans are striving to adhere. Murray's complaint against upscale liberals is not that they are libertines but that they fail to preach what they practice.
What light does this shed on the Republican race? For starters, Mitt Romney is literally from Belmont, where he raised his family in a large house and now has a condominium, and where he helped build a large Mormon temple visible as you drive by on Route 2. He grew up in another Belmont, Bloomfield Hills, Mich.
Rick Santorum, in contrast, describes himself as the grandson of an immigrant coal miner, a product of Fishtown. He often tells of getting his political start winning two House races in the steel mill suburbs of Pittsburgh.
When you look at where these two candidates are drawing their strongest support, you see a similar contrast.
Romney in 2008 and this year has run consistently strongest in the high-income, high-education Belmonts. Santorum has run strongest in areas with more downscale Republican voters.
Both trends are politically problematic. It is not attractive for Republicans (though it can be for Democrats) to advertise a candidate's appeal to affluent voters.
But it's also true that there just aren't so many voters in Fishtown anymore; voter turnout there is way down. Pittsburgh is our one major metro area with more deaths than births. There aren't as many neighborhoods filled with devout Catholics with large families as there were 50 years ago.
Republicans are choosing between a candidate from a Belmont that's doing just fine and one who claims ties with a Fishtown that isn't what it used to be. Not an ideal choice.