[M Kehoe]

Is it legitimate for the Republican-controlled Senate to refrain from confirming a replacement for the late Supreme Court justice until a new president is elected,

Yes.

Or does the Senate have an obligation to approve a qualified nominee put forth by President Obama,

No.

That was easy. What do I win?

5.56mm

[Leaning Right]

Right you are. The President can nominate whomever he wants to, for any reason. And the Senate can deny confirmation, for any reason.

The Senate is under no obligation to approve. This is not a dictatorship (yet).