Right now, Barack Obama and the GOP Senate are in a standoff regarding the replacement of Justice Antonin Scalia. As part of their justification for refusing to consider Obama's nominee, Senate Republicans have begun to argue that an unwritten rule prevents the Senate from considering any nominees that would replace a vacancy that occurs during an election year.
I had originally planned to write a quick post running through every time in the past that a Supreme Court vacancy has appeared during an election year. However, when I put together a chart of every justice, arranged by the year they left the bench, I was immediately struck by just how few vacancies have occurred in election years in the past century. You can see the chart below, with the election-year vacancies highlighted in yellow.
If things were happening randomly, 25% of vacancies should occur during election years. Beginning in 1893, there have been 61 vacancies on the bench. Of these, four -- four! -- have occurred during election years (with one of those four, Charles Evans Hughes, resigning to run for president in the 1916 election). That's just over 6.5%. By contrast, prior to 1893, there were 45 vacancies of which 11 came during election years, or just over 24%.
The justices have even managed to keep the reaper at bay: beginning in 1893 there have been 21 deaths on the bench, but only two, Scalia and Joseph Rucker Lamar, during an election year (9.5%). Prior to 1893, again, just over 24%. (Since 1983, the number of deaths during each year of the presidential term: 8,8,3,2.)
You'll note that until around the turn of the last century, the vast majority of justices left the bench through death. After the turn of the century, most justices left the bench through retirement. Why is this? Well, I'd wager it's because until 1869, judges did not receive pensions after leaving the bench, giving them a strong incentive to stick around until God retired them. Also, with modern medicine there's probably more justices living past the point where they feel they're losing their energy or mental edge, and who choose to step down. Maybe there's just been a cultural shift around the idea of working until you're dead -- have to ask someone much older than me.
Whatever the cause, with most justices now making a deliberate choice of when to leave service rather than leave it to fate, it makes sense they would want to avoid vacating during an election year precisely to avoid the type of chaos brought by the death of Antonin Scalia.