My practice prior to being county attorney was primarily civil transactional work. I now have a potential defendant for whom I performed such services. Does this present a conflict?

As a general rule, no, there is no conflict as long as:

(1) the criminal case does not involve the same legal matters you previously handled for that ex-client, and
(2) you don't use against that ex-client any confidential information you learned about the ex-client from that prior representation.

For more, see pp. 58-68 of our "Conflicts of Interest" book authored by Chip Wilkinson, which you should have received a few months ago.

Despite Shannon's admonition, the safe thing to do is have the Court appoint an Attorney Pro Tem to handle the case. Why waste time with a grieveance later from this former client when a simple procedure can save grief?