They cannot violate the Constitution in this matter, because they cannot prevent Obama from making a nomination. Of course, Obama cannot also force them to approve his pick.
========
Correct. He cannot FORCE THEM.
However, as President, he CAN call a weekly press conference and show charts of how long it has taken to approve each previous nominee and DRAMATICALLY show what assholes the Republicans are being and how they are violating the Constitution by refusing to approve any nominees and some of them are on RECORD as saying they will not approve ANY nominee. But we all know that if he nominated an ULTRA-CONSERVATIVE they WOULD approve that one.
And the Democratic party can, and should, file Impeachment charges against any Senator who has publically stated he would not approve any nominee because he is violating his Oath of Office as well as the Constitution.