The practice of the Senate not confirming Supreme Court nominees in an Election Year has not been made up by Republicans in response to Justice Scalia's untimely passing.  It is a rule first outlined by the Democrat poised to replace Sen. Harry Reid as Minority Leader.  

In July 2007, fully 18 months before the end of President George W. Bush's second term of office, Sen. Chuck Schumer advocated blocking any further Bush nominees to the Supreme Court:

[F]or the rest of this President’s term . . . :  We should reverse the presumption of confirmation.  The Supreme Court is dangerously out of balance. . . . Given the track record of this President and the experience of obfuscation at the hearings, with respect to the Supreme Court, at least:  I will recommend to my colleagues that we should not confirm a Supreme Court nominee EXCEPT in extraordinary circumstances.

As we noted in Part 1 of this series, in modern times, Supreme Court vacancies that occur in election years are traditionally not filled until after the election, giving the people an opportunity to speak on the issue.  The Senate holds the constitutional power to advise and consent on the president's nominees, but per Sen. Schumer's own rule, the Senate should not confirm a potential Supreme Court justice nominated by a lame-duck president.