The FBI probe of House Speaker Christopher G. Donovan’s campaign finances has unleashed a torrent of work for seven criminal defense lawyers, who are already busy representing two former political staffers and five “roll your own” tobacco merchants caught up in the investigation.

As the list of potential witnesses seeking legal counsel grows, the top White Collar defense lawyers in the state face a challenge of both chance and skill — how to end up representing a “big fish” client — with big-ticket legal exposure, instead of a minnow or no client at all.