Chris Tidland had the round of the day on the Senator Course at Capitol Hill with a second-round 64. Tidland finished alongside Doug LaBelle II in a tie for fourth at 9-under-par 135.
Duke, who is currently 42nd on the Nationwide Tour with a chance, albeit slim, to earn his way onto the PGA Tour by the end of the week, picked up his first birdie of the day at the par-5 fifth.
He added a birdie at the par-5 eighth and landed his approach inside three feet for a birdie at the ninth.
Duke continued to plug along on the back side with a 16-foot putt for birdie at the 12th and collected a birdie at the 15th after his second shot stopped eight feet from the cup.
The 34-year-old took the outright lead at the par-5 17th with a tap-in birdie after a long eagle attempt almost found the bottom of the hole.
'I've got a really good rhythm going,' said Duke. 'I've been sick the past week so I'm just going off of momentum right now. I had some good numbers and hit some good shots with the wedge.'
Couch, who won this year's Oregon Classic, dropped his tee shot within eight feet for a birdie at the par-3 seventh and followed that up with a birdie at the very next hole.
The 30-year-old birdied the 10th to reach 9 under and hit his second shot to six feet for a birdie at the par-4 14th.
At the par-5 17th, Couch drained a five-foot putt for his fifth birdie of the day to join Duke atop the leaderboard.
'I was a stress case last year but I can relax this week,' said Couch, who posted a 67 on Friday. 'It was a very stress-free round today. Hopefully I can keep doing this for 36 more holes.'
Brigman had a rocky front side with three birdies and three bogeys but he caught fire around the turn to stay within reach.
The 27-year-old birdied the 10th and added a birdie at the 13th to reach minus-8.
Brigman then birdied the 15th and converted a six-foot putt for a birdie at the 17th.
At the par-4 18th, Brigman had a putt from the same distance to join the leaders but his birdie attempt missed right.
'Obviously there are some really good scores out there,' said Brigman, who shot a 68. 'You're going to have to play pretty darn good on the weekend to win. A lot of guys want it too.'
Bo Van Pelt and Jason Dufner shared sixth place at 8-under-par 136.
First-round leader Zach Johnson managed an even-par 72 to join Jess Daley and Tjaart van der Walt at 7-under-par 137.
Wes Short was one shot further back at 6-under-par 138.
The top 20 on the money list after the Nationwide Tour money list will earn their PGA Tour cards. Those who finish from 21-55 will secure their playing position on the Nationwide Tour in 2004.