The round was completed despite an 85-minute weather delay.
Natalie Gulbis, the first-round leader, stumbled to a 5-over 41 on the back nine to end alone in third at 3-under-par 213 after a round of even-par 72.
Stacy Prammanasudh and Catriona Matthew are one stroke further back at 2-under-par 214. Karine Icher is the final player in red figures at minus-1.
Ward opened the third round three strokes behind Koch, but quickly made her move. Ward rolled in a 12-foot birdie try at the second. She moved to 6 under with a birdie on the par-5 fifth.
The 32-year-old Ward caught fire late on the front side. She birdied the par-5 eighth and came right back with a 20-foot birdie putt on the ninth.
Ward, who won for the fourth time on tour last week, made it three straight as she birdied the par-4 10th from 6 feet out on Tres Marias Residential Golf Club.
After the weather delay, Ward tripped to a bogey at the par-3 14th to slip back to minus-eight, one shot behind Koch. Ward forged back into a share of the lead with a 15-foot birdie putt on the 15th.
Ward parred her next two holes, but slipped back into a tie for the lead at minus-8 as she stumbled to a bogey at the last.
'I am excited. It is something that I have always put forth as a goal,' said Ward of her chance to win two straight weeks. 'It was so long since my previous win until last week. I was telling the press last week that I have never played the week following a victory so this is really a neat opportunity that I have.'
Koch was hot early on the front nine as well. She converted consecutive birdies from the second to get to 9 under. The 34-year-old picked up another birdie on the seventh from 4 feet out, but stumbled to a bogey on No. 8 after driving into a creek of the tee.
The Sweden native stumbled to a bogey on 13 after the weather delay. Koch rebounded with a 12-foot birdie putt at the 14th to get back to minus-9. She tripped to another bogey at the 17th after hitting her approach over the green before a par at the last.
'I think overall today I played pretty steady,' said Koch. 'I hit a lot of good drives. I did not play quite as steady as the first two days, but overall pretty good. There were a few putts that I let go today. I hit a few bad putts and hit a few short. If I could have made two or three more putts, then I would be quite happy with my round.'