Fergus had a 36-hole total of 7-under 137.
"I'm in a position we'd all like to be in going into the last round," said McNulty, who shot a 66. "I'm delighted with my round and look forward to tomorrow."
Gary McCord had a 66 to tie D.A. Weibring for third place at 4-under 140. Mike Reid was four shots back at 141 and first-round leader Isao Aoki was sixth at 142.
Bob Gilder, Lanny Wadkins, Don Pooley and Des Smyth share seventh place at one-under-par 143.