[tsomer]

Who replaces him.
Will Cuomo pick?

[onyx]

Nope. Special election.

[KC Burke]

Generally when Senators (not Congressmen) leave office during a term, they are subject to a replacement by appointment under their state's laws and then that replacement is replaced in a special election or the next regular election for that six year post.

Congressional Representatives, such as Lee, are sometime replaced in a special election called if their is a long portion of their term left, or if the portion is brief, the post can be left vacant for a month or two. I imagine that a special election will be called in the next two weeks if he has officially resigned.

[RedMDer]

Congressmen can only be elected. There will be a special election.