France's Delacour, whose first-round 63 had given her a one-shot lead, again took advantage of the rain-softened Columbia Edgewater course in Portland, Oregon.
Her five birdies included four in five holes in her final nine - at the third, fifth, sixth and seventh.
She nearly holed an eagle putt at the par-five seventh, and a birdie putt at her final hole, the ninth, was just inches short, but her closing par left Delacour on 14-under par 130 - one stroke clear of Sweden's Linn Grant.
American Megan Khang, who broke through for a first LPGA title in Canada on Sunday, was a further stroke back after an eagle and five birdies in a six-under 66 for 132.
Delacour acknowledged that chasing a first major title of her 10-year LPGA career wasn't uppermost in her mind coming into the week as she continues to wrestle with mental health issues that prompted her to take a break from the game last year.
One problem, she said, was the feeling that "if I play bad golf, I'm a bad person and if I play good golf I'm a good person.
"So that's what we've been trying (to work on) with my team," she said. "You're a good person no matter what your round of golf. So that's mainly my goal."
With that in mind, Delacour wasn't placing too much emphasis on a bogey-free two days.
"I'm a leader on the LPGA, but golf is just golf," she said, adding that talking more about her struggles had proved therapeutic.
As for the weekend, Delacour said she'd just try to keep doing what she'd been doing.
"We keep the same plan," she said. "We keep the same game plan no matter what and we're just going to try our best."
Plenty of players remained in striking distance, with eight players tied for fourth on 10-under par 134.
That group included China's Yin Ruoning, who charged up the leaderboard with an eight-under par 64 that featured eight birdies.
Delacour's closest pursuer, Grant, was relishing the tree-lined Columbia Edgewater layout, saying the hemmed-in fairways made it easier to focus.
She was also enjoying the benefits of a course playing so soft that she can "just kind of hit the numbers and get the ball to and spin back.
"Just like yesterday, (I'm) kind of feeling like every hole is an opportunity and kind of not get bummed out when I don't get the birdies, hole the putts," she said.
"It's a lot of chances, which is fun," added Grant, who won her first LPGA title in Ohio in July and has had four top-20 finishes since.
Khang had an eagle on her card for the second straight day. After chipping in at the par-five seventh on Thursday, she reached the green at the par-five fifth on Friday and poured in her eagle putt.
