If you are looking for someone to blame for the polarized nature of our politics today, here are two nominees: Franklin Delano Roosevelt and the political science establishment.

Together they set out the argument for the situation we have in Washington — a Republican Party loaded with conservatives, a Democratic Party larded with liberals, and few in between. The result has been gridlock, rancor and a sense of despair if not hopelessness in the capital and across the country.

We have a political landscape where it is possible to argue that the most conservative Democrats in Congress today are more liberal than the most liberal Republicans. There is virtually no overlap, no real party dissenters of the sort who were unacceptable to FDR, who wanted a party of ideological purity, and who were inexplicable to political scientists, who looked longingly at the ideologically disciplined parties in Europe and wondered why American parties so defied logic.

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.

The most prominent advocate for ideologically aligned parties was Roosevelt, who once told Sam Rosenman, a White House speechwriter and the first White House counsel, “We ought to have two real parties — one liberal and the other conservative.”

FDR set out to create just that with his effort to purge conservatives and New Deal foes from the Democratic Party. He singled out, among others, Walter F. George of Georgia, Ellison D. “Cotton Ed” Smith of South Carolina and Millard Tydings of Maryland, all of whom prevailed against the onslaught of White House opprobrium.

Susan Dunn, a Williams College historian who has written the definitive account of the Roosevelt offensive, said the president’s biggest blunder “was to undertake the purge in the absence of impressive challengers to conservative incumbents.”

That very likely is true. For whatever reason, the mushy party system prevailed — and had unforeseen consequences even for Roosevelt. Many of the most ardent opponents of the New Deal turned out to be the most ardent supporters of the president’s initiatives in foreign affairs, supporting Roosevelt on Lend-Lease, so much so that party alignment was doomed as World War II approached.

It gained new life a dozen years later, however, when the American Political Science Review published a landmark article called “Toward a More Responsible Two-Party System,” which argued that American parties needed “sufficient internal cohesion” and a “degree of unity within the parties” that they lacked at mid-century. At that time, the Democratic Party had such conservatives as Sen. Harry F. Byrd Sr. of Virginia, Sen. Strom Thurmond of South Carolina and a series of Southern committee chairmen. The Republicans Party had such liberals as Gov. Earl Warren of California, Rep. Clifford P. Case of New Jersey and Sen. Henry Cabot Lodge Jr. of Massachusetts.

The political scientists’ report echoed scholarly critiques dating back a half century, when important figures like Woodrow Wilson, then a prominent political scientist, and Herbert Croly, an important thinker in the Progressive movement and the co-founder of The New Republic, raised questions about the American party system.

“However one may deplore that system, he must concede that it has displayed, if nothing else, a very impressive ability to survive,” Austin Ranney, then a political scientist at the University of Illinois and later the chairman of the political science department at the University of California at Berkeley, wrote in a contemporary critique of the 1950 report.

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.

A recent National Journal study showed that every Republican member of the Senate has a voting record to the right of every Democratic member of the Senate, and that only five House Republicans have a voting record to the left of Rep. Gene Taylor of Mississippi, the Democrat with the most conservative voting record. The journal has been conducting these studies since 1982. Only once before, in 1999, did the Senate have a profile like it does today.

In an important retrospective on the 1950 political scientists’ report published on its 50th anniversary, UCLA political scientist Barbara Sinclair argued that the modern parties “do represent a clearer policy message than they did 50 years ago.”

She’s right. If you vote for a Republican today, you are very likely voting for a conservative, and if you vote for a Democrat you are very likely voting for a liberal. That’s clear. One other thing also is clear: The political system is a lot worse off.