Come January, America will have a Democratic president and a Republican Speaker of the House of Representatives, a rare configuration in the 150-plus years that there has been a Republican party. There were Republican House majorities during Democratic administrations for two years after the 1894, 1918, and 1946 midterm elections, and for six years after the 1994 midterms.

In 2011, only the memory of the acrimonious cohabitation after 1994 will matter. The Republican speaker who took the gavel after that election had famously disparaged a leading senator from his own party as the “tax collector for the welfare state.” Republicans swept …