Who Will Challenge the Senate Defectors?

Ongoing fluidity notwithstanding, it seems safe to assume that once their heads stop spinning, the state (and maybe national) Democrats will make it a serious priority to make both Pedro Espada and Hiram Monserrate into former elected officials.
The senators, who cost the Democrats support of the State Senate by crossing the aisle to provide the 31st and 32nd votes to Republican leader Dean Skelos, come from overwhelmingly Democratic districts and will both, in theory, be highly vulnerable to party-backed primary challenges.
But, assuming Espada and Monserrate are still around next year–and that they haven’t cut some sort of deal with the party by then to flip back–who will do the challenging?
Here, with the help of some smart readers, are a few speculative candidates.