Pragmatists want to change the GOP so that it can win elections and govern effectively. Tea party Republicans prefer to express their principles regardless of consequences, which is why the Pew survey in September found that 71% of them favored a government shutdown even though nearly 40% of them expected that shutdown to have a “major” impact on the economy.

Third-party threats frighten Republican leaders. They remember that Ross Perot’s independent challenge badly hurt George H.W. Bush’s re-election campaign in 1992.

Canadian conservatives were locked out of power for nearly 15 years by a party split in the 1990s. British Conservatives fear that a rise in support for the United Kingdom Independence Party could drain support from Britain’s Conservative-Liberal governing coalition.

Yet politics is a complicated business, and it’s not always true that a party is weakened by the departure of its most extreme supporters.