Republican presidential candidates must work harder for their electoral votes than their Democrat opponents, yet they have a statistically significant advantage over those same opponents. What do I mean by "work harder", what is the statistically significant advantage, and what are its implications for future elections?
Working Harder -- A Result of Long-Term Political Realignment
From the election of 1880 -- the first post-Reconstruction election -- through the election of 1928, the percentage of electoral votes cast for the Republican candidate was usually the same as, or greater than, the percentage of States won by the Republican candidate. That relationship reflected the tendency of Republicans to win the more-populous States of the Northeast and Upper Midwest, whereas Democrats could only count on the less-populous States of the South.
After the aberrant election years of 1932-1948 (spanning the Great Depression, World War II, and the Dixiecrats), the relationship shifted, and the realignment of party allegiances began. Eisenhower made inroads into the "Solid South" in 1952, and greater inroads in 1956, while holding onto traditional Republican States. Then, as the Northeast and Upper Midwest began increasingly to vote for Democrats, the South began increasingly to vote for Republicans. This realignment was complete by the election of 1980, when the Democrat (Carter) won only one Southern State -- his home State of Georgia.
Although the population of the Southern States has grown faster than the population of the States in the Upper Midwest and Northeast, the net result of realignment, thus far, has been to the disadvantage of Republican candidates. That is, since realignment Republicans must win a higher percentage of States than they did before realignment in order to win a given number of electoral votes. That relationship will change, of course, as realignment persists and the South continues to outstrip the North in population growth. But it holds for now, even in the aftermath of the 2000 census.
The Statistical Advantage
Republican presidential candidates, in spite of their geographic disadvantage, have held a significant statistical advantage over Democrats since the 1950s. Perhaps it began with Eisenhower, survived the Goldwater debacle and Nixon's disgrace, was renewed by Reagan, and wasn't diminished by Clinton's ephemeral and largely partisan appeal. Whatever the explanation, the share of Republican presidential candidates has been out of proportion to their share of the popular vote, which means that they have tended to do better than Democrats in populous swing states.
The effect of this phenomenon is shown by a statistical analysis of the percentage of electoral vote going to the winner of elections from 1956 through 2000. The best regression equation has only two significant explanatory variables: percentage of two-party popular vote and Republican Party affiliation.
If Republicans can hold onto their solid base of States in the South, Southwest, the Plains, and the Rocky Mountains -- and if they do not destroy the trust in presidential Republicanism that seems to be the legacy of Eisenhower and Reagan -- they can win the White House more often than not for decades to come. That's not a guarantee, because those are big "ifs".