The usual methods of co-optation by which Republican and Democratic politicians have maintained cohesion within their respective big tents are proving remarkably ineffective.

GOP candidates’ concessions to the right only seem to feed the demand for more; ditto for the attempted leftward movements not only of Clinton but also of former Maryland governor Martin O’Malley.

In short, a lot of the energy in politics now comes from those who reject, well, politics, at least politics as we know it. They insist on simple solutions to complex problems — whether it’s Sanders’s call for free state college tuition, paid for by a tax on stock traders, or Trump’s promise to build a wall along the Mexican border, paid for by Mexico.

The likely scenario for 2016 is that Republicans and Democrats will hang together, though the specific ideological agenda that emerges from their respective primaries is still very much up for grabs. The incentives to avoid actual party crack-up are overwhelming; and the resources at the disposal of establishment Republicans and Democrats remain formidable.