grammaticality - 'Alert whomever may read' - should it be 'whoever'?

I read this morning :

... words which leap from the second verse and alert whomever may read the epistle ... to the fact that ...

I am not clear with what exactly is going on with 'alert whomever may read'. Is 'whomever' the grammatical object of 'alert' or is it the grammatical subject of 'may read' ? Or is it both ?

So should it be 'whoever' ?