American federal elections happen every two years. There is simply no convention against Supreme Court nominations in those years.
And if the Senate did reject the most Republican Democrat that President Obama could find, what then? No President entering office in January is going to be any friend of Mitch McConnell's.
Not only is there is every reason to assume that Bernie Sanders will be on the ballot with or without the endorsement of the Democratic Party, but look who else would be.
What if President Clinton nominated Barack Obama? Or anyone at all, come to that? What if President Trump nominated Hillary Clinton? Or anyone at all, come to that?
Senate Republicans need to get Merrick Garland confirmed.
And thank their lucky stars that this vacancy has arisen this year rather than next.