Source: The Constitution of the United States ("shall" emphasis added)
As quoted in If Republicans block Obama’s Supreme Court nomination, he wins anyway. (Washington Post, 2/14/2016)
But the GOP might soon reconsider if they see the implications of refusing to allow Obama to replace Scalia: A divided court leaves lower court rulings in place. And the lower courts are blue.
Yes, President Obama Can Still Nominate a Supreme Court Justice. (Nation, 2/14/2016)
Dear GOP, Where in your revered Constitution does it say that the President shall wait until after an election to make Supreme Court appointment? (2/14/2016)