Back in 2007, Bernie Sanders opposed the comprehensive immigration reform legislation pushed by Sens. John McCain and Ted Kennedy and backed by then-Sens. Hillary Clinton and Chuck Schumer. His main focus of opposition at the time was on how importing guest workers lowers American wages. And when Clinton challenged him about the 2007 bill in a debate, he returned to the guest worker point from a slightly different angle, citing a Southern Poverty Law Center report that described federal H-2A and H-2B visa programs as "close to slavery."

Under the circumstances, when you learn that Sanders in the recent past co-sponsored a bill called the "H-2A Improvement Act" you might assume it was a bill to improve working conditions for H-2A workers.

But in fact, as United Farm Workers president Arturo Rodriguez points out while detailing a larger bill of particulars against Sanders, what the bill actually did was simply expand the existing H-2A program to allow dairy farms to hire H-2A workers — who are currently restricted to use in seasonal agriculture.

Sanders's policy director, Warren Gunnels, tells me the H-2A program offers "much stronger safeguards for workers than the H-2B program," and that Sanders's real concern is not with the two H-2 programs condemned in the SPLC report, but specifically with the H-2B program and with the H-1B program for white-collar technical workers. The SPLC itself is not nearly so sanguine about the practical merits of the H-2A worker protections, saying "far too many of the protections — as in the bracero program — exist only on paper." Indeed, Gunnels himself concedes that the program "is in serious need of reform" due to a lack of enforcement.

So whether you draw the distinction or not, you have the reality that Sanders co-sponsored a bill to significantly expand the scope of a program that he regards as deeply flawed and that a report he cited deems close to slavery. The question is why?

Dairy state politicians like to help dairy farms

Rodriguez raises the H-2A Improvement Act to try to raise a larger concern about Sanders and immigration, but what it really illustrates is a larger point about interest group politics and the limits of ideology. This was not a very popular bill in the Senate, and the list of its other Democratic co-sponsors is telling — you've got Chuck Schumer, Herb Kohl, Kirsten Gillibrand, and Pat Leahy. That's not an ideological bloc or a "faction" of the Democratic Party. It's Democrats from states with a lot of dairy farms, and they are carrying water for the dairy industry.

Now, New York state obviously has a much bigger and more diverse economy than Vermont's, so in addition to doing favors for the dairy industry New York's congressional delegation tends to look sympathetically on the policy needs of Wall Street. They're not entirely in the tank. Both Schumer and Gillibrand backed Dodd-Frank and continue to defend most of its provisions. But certainly around the margins, Schumer is someone financial reformers have a lot of problems with. In much the same way, Elizabeth Warren is left-wing on almost everything but is also a big ally of the medical device manufacturing industry, which is big in Massachusetts. It's also big in Minnesota, and Al Franken also has the industry's back.

To bring this back to the dairy farmers, unlike Rodriguez I don't particularly think Sanders needs to answer any tough questions about why he agreed to co-sponsor a dead-on-arrival piece of legislation that's backed by an important home-state industry. It's totally obvious why he did it — it was important to a home-state industry!

But one key fact about Sanders's political persona is that Vermont just doesn't have many home-state industries. There are no big cities in the state, and there are no suburbs of a big city in the state. So Sanders doesn't get a lot of calls from hometown industries asking for favors. But when he does get asked favors from the dairy industry, he seems inclined to see if he can be helpful.

Special interest clout isn't just about money

The problem for Sanders's political revolution is that even if you could elect dozens of Sanders-like politicians to the Senate, you can't create dozens of states that replicate the political economy of Vermont. If you want to save tens of billions of dollars by shifting to a single-payer health care system that drastically reduces payments to the pharmaceutical and medical device industries, you have to do so over the objections of the legislators who represent those industries' home turf. If you want to break up the big banks, you are going to hear objections from legislators in the tri-state area.

In his official rhetoric, Sanders tends to attribute industry power to the campaign finance system and the influence of billionaire donors. But I don't think he supported the dairy industry because of campaign contributions from billionaire donors. Among other things, I doubt Vermont is full of billionaire dairy barons. I think Sanders supported the dairy industry for the banal reason that most legislators think it's beneficial to their constituents to support the growth and prosperity of local industries. And if he manages to become president, he's going to find that this dynamic is a big problem for his legislative agenda.