They will have to approve someone. And no matter who it is, no mater how moderate, the conservative base will view it as Republicans caving to Obama, ....again.
It is nearly a no win scenario for the Republicans. If they try to delay this for A YEAR that will energize the Democratic base, which is already on fire for Bernie, to turn out in large numbers. All they have to do is remind the African American voter, "these Republicans are trying to destroy President Obama's legacy!".
And, all Obama has to do is nominate a left-leaning moderate hispanic or a woman. If the Republicans oppose they lose that vote as well.
There are few win scenarios for conservatives on this.