But today, FDR and the political science establishment having had their way, the United States has its most ideologically aligned party system in modern history — and perhaps the biggest political crisis in modern history.

Party caucuses always have reinforced party discipline, but for the first time both caucuses are enforcing ideological discipline as well. In the course of their work, lawmakers almost never encounter views that depart from their own, almost never form friendships with their political adversaries. If they don’t practice ideological compromise inside their own parties, they are less likely — less able — to practice it on the floor of both houses of Congress.

“We finally got ideological purity, and it’s a disaster for the country,” says former Gov. Angus King of Maine, an independent. “We have ideological gridlock. You can’t solve problems this way.”

Indeed, the lack of a middle in the American political class is the American problem. The irony is that the American problem repeatedly has been held up as the American solution…

One reason the old system survived for so long is that the multiplicity of interests and ideologies inside American parties imposed the sorts of restraints on the majority that Americans liked, much like the checks and balances and separation of powers designed in the Constitution to protect the rights and viewpoints of the minority.

Now we have just the kind of political-party system Roosevelt and the political scientists envisioned. We are living the future, and it does not work.