In the two decades since then, Democrats have won four of six presidential elections (and won a plurality of the popular vote in a fifth) and Republicans have won majorities in the House in eight of the last 10 elections. And these results have occurred despite a marked increase in straight-ticket voting. Their margins, however, have been smaller than the other parties’ margins in the pre-1990 period. The average Democratic popular vote margin in presidential races has been 4 percent, not 10 percent. And Republicans from 1994 to 2012 never won as many seats in the House as Democrats did in all 18 elections between 1958 and 1992.

In the years since the Clinton and Gingrich breakthroughs both parties, I would argue, have tended to compete competently under circumstances in which there was considerable uncertainty about which one would prevail. In every presidential election since Clinton was re-elected in 1996, there has been uncertainty about the outcome (even in 2008, when John McCain led Barack Obama in polls during the first half of September; though some would argue the point). And in every House election cycle with the exception of 2008 and the possible exception of 2012, there has been considerable uncertainty whether the party in power would retain its majority.