Peter Wilson only managed a 1-over 71, but is alone in second place at minus-3. Stephen Gangluff and Michael Harris both shot 71s on Friday and are tied for third place at 2-under-par 138.
Tomasulo trailed first-round leader Ben Bunny by three. Tomasulo, an All- American at the University of California (Berkeley), benefited as Bunny hopped down the leaderboard.
Tomasulo bogeyed the sixth, but drained a 25-foot birdie putt at the eighth. He tallied birdies at the 14th and 15th holes to reach 4 under par for the championship.
He rolled in a 15-foot birdie putt at the 18th to move two clear of the field.
'Man, it was playing tough out there,' said Tomasulo. 'I was able to keep my ball flight low, which is what you have to do on a day like this. It's a great course that sets up well for me. This place doesn't let up at all.'
Tomasulo is a tour rookie who has had some success thus far in his first season. He won in his home state at the Long Beach Open, and he sits fifth on the Canadian Tour money list. All that's missing is a Canadian Tour victory.
'I've been playing well most of the year, and my win back home gave me a lot of confidence,' said Tomasulo. 'I just want to keep it going and see where I am at the end. Hopefully, I can bring it home.'
John Ellis (69), Matt Hansen (69), Anders Hultman (70), Wes Heffernan (68) and Rob Oppenheim (72) share fifth place at 1-under-par 139. Matt McQuillan (68) and Brad Fritsch (67) are tied for 10th at even-par 140.
The 36-hole cut fell at seven-over-par 147 and 63 players made it to the weekend.