Obama's out to lunch, however, when he claims that there is no history or "unwritten law that says that it can only be done on off-years." It's true that nominations have been made and considered in election years, but the fact is that no lame-duck president has filled a vacancy that arose in the election year of his successor.
Of course, we've only had "lame-duck" presidencies since the 22nd Amendment was ratified in 1952 limiting presidents to two terms. Before that, presidents have had election-year vacancies filled only if their party controlled the Senate, or by appointing a justice of, or friendly to, the opposition party.
Obama's own vice president, Joe Biden, was closer to the historical norm when, in 1992, as a senator from Delaware, he advocated a Democratic roadblock to any nomination by then-President George H.W. Bush.
Obama's faithful mouthpiece, the New York Times, provided a useful graphic of presidential nominations from George Washington to the present, with the utterly misleading headline "Supreme Court Nominees Considered in Election Years Are Usually Confirmed."
The last time that happened was in 1932, when Republican Herbert Hoover was a de facto lame duck and the Democrats controlled the Senate.
Two years earlier, a coalition of Democrats and progressive Republicans rejected John J. Parker — the only nominee rejected between 1890 and 1968. Hoover then chose Benjamin Cardozo, perhaps the most prominent progressive jurist in America. Since he was replacing the progressive icon Oliver Wendell Holmes Jr., Hoover thought Cardozo should get the seat to "be fair to all elements." (Justice Willis Van Devanter correctly warned Hoover that the Democrats would not be so fair once they were in the White House.) This would be the equivalent today of Obama nominating Republican Sen. Ted Cruz of Texas to the high court.