This latest episode in the endless Republican reality show is not chiefly about the incompetence and incessant squabbling of ideologues and petty politicians, although it's that, too. Nor is it the outcome of the intense partisan polarization that has thrown Washington into gridlock, as if the problem is abstract partisanship itself, with Democrats and Republicans equally at fault. Least of all is it about rescuing the economy from the Democrats' profligate deficit spending, as Republicans claim – not with the deficit shrinking to its lowest level since the financial disaster of 2008 and with the outlook improving. This crisis is about nothing other than the Republican Party – its radicalization, its stunning lack of leadership and its disregard for the Constitution.

The Republicans have now joined a relatively small number of major American political parties that became the captive of a narrow ideology and either jettisoned or silenced their more moderate elements. The Democratic Party suffered this fate in the 1840s and 1850s, when Southern slaveholders took command of the party's levers of power. So, temporarily, did the Republicans in 1964, when Barry Goldwater's presidential campaign claimed the party for extremists on the right, an augury of things to come. But today's Republicans, whatever their pretensions about channeling the Founding Fathers, are so contemptuous of American history and institutions that they cannot learn from even their own recent past.

Like earlier declines into dogmatic politics, the Republicans descended gradually, beginning with Ronald Reagan's departure from the White House in 1989. Reagan had governed shrewdly. While getting his way on what he thought was important, including dramatically lowering marginal tax rates and combating the Soviet Union, he knew how to compromise. He also knew how to exploit the culture wars, paying lip service to causes like the "pro-life" movement without risking any political capital on them. Reagan adroitly kept his true- believer supporters in line even as he raised taxes no fewer than 11 times, raised government spending by 57 percent (in current dollars), and nearly tripled the national debt to $2.6 trillion. Yet while Reagan's success continued to shape national politics for decades after he left office, he alone proved capable of holding together the conservative coalition that had swept him to power....