Chalmers, the overnight leader, had a 1-under 71 and joined Guetz at 12-under 204. They shared a one-shot lead on John Riegger, who carded a 69 Saturday to get to 11-under 205.
Tom Gillis had a 68 and was in fourth place at 10-under 206, while Ron Whittaker (70) and Jeremy Anderson (73) were a shot further back at 207.
Including this week's tournament, there are just three events remaining until the season-ending Nationwide Tour Championship. Players must be inside the top 25 on the final money list to receive PGA TOUR cards for next year.
Chalmers is currently No. 35 on that list, while Guetz is languishing at 237th. Both players are seeking their second career Nationwide Tour victories.
More importantly for Guetz, he is all but guaranteed a top-25 finish on Sunday. With no status on TOUR, he needs that to gain entrance to next week's Chattanooga Classic.
'Monday morning, the goals were to qualify for the tournament, make the cut and finish in the top 25,' said Guetz, 33. 'It looks like I should go ahead and add trying to win the tournament to that list.'
Guetz didn't make his first par Saturday until the fifth hole. He opened with a double-bogey at the first after pulling his tee shot, then went birdie-bogey-birdie over the next three holes.
He made another birdie at the seventh, and two in a row from the 12th. Guetz birdied the 17th hole and held the lead by himself, but a closing bogey at the 18th dropped him back to 12 under.
It was a good day overall, especially with winds gusting to 30 mph.
'It was a day of survival out there with the wind,' said Guetz. 'It feels good that I was able to get through today and I'm happy where I am.'
Chalmers joined Guetz at 12 under with a birdie at the par-4 18th, his third birdie on the back nine. He had made two bogeys without a birdie on the front nine after carrying a one-shot lead into the third round.
'I felt like I kept dodging bullets all day,' said Chalmers, 34. 'I made up-and-down after up-and-down on the front ... I was real proud of the way I rebounded to shoot 3 under on the back in these conditions.'
The second round was completed Saturday morning after being suspended Friday night due to darkness -- an inevitability after the first round was pushed back by a weather delay on Thursday.