Ranked 130th in the world, Dahmen came agonizingly close to finishing the day above the two-time major winner as the outright leader but missed his birdie putt at the 18th hole by mere inches.
The 34-year-old, who secured his first and only win on the PGA Tour in March 2021 with victory at the Corales Puntacana Championship, carded a two-under 68 to add to his opening round 67.
Having admitted to almost skipping the event qualifier, the American reflected on a “whirlwind” two days, when he has rapidly become a crowd favorite in Brookline, much to Dahmen’s surprise.
“It is unbelievable to me how many people know my name or yell for me out there,” Dahmen told reporters.
“It’s weird. I’m getting recognized a little bit more off the golf course … my wife will look at me, like, ‘What is happening?’
“It’s not normal. I don’t know if I’ll ever get used to it, but it comes with good golf.”
Morikawa, pursuing a third major in three years after triumphs at the PGA Championship and the Open, starred with a round-best 66. The world number seven made a scintillating start to his second round, sinking three birdies en route to a three-under score through his first nine holes.
A bogey on the fourth hole – his 13th – provided the sole blemish on the 25-year-old’s scorecard, with Morikawa ending the day in a confident mood.
“No one has taken it deep so far and run away, but right now my game feels really good,” Morikawa told reporters.
“The last few days is a huge confidence booster for me heading into this weekend, and hopefully we can kind of make some separation somehow.”
Defending champion Jon Rahm and four-time major winner Rory McIlroy headline the five players in hot pursuit of the duo, one stroke off the lead after shooting 67 and 69 respectively.
McIlroy bounced back superbly from a double bogey on the third hole, which could have been markedly worse. Finding himself in the long grass after an errant approach shot, disaster struck when it took the Northern Irishman three shots to escape – the ball twice getting stuck in the fescue.
It led to the rare instance of the 33-year-old fist pumping in celebration to a double bogey, avoiding a triple with a superb 22-yard putt.
“I stayed patient, and I knew I was going to give myself chances if I just hit the ball the way I have been hitting it,” McIlroy told reporters.
“Today was a really good example of just having a good attitude.”
American trio Hayden Buckley, Aaron Wise, and Beau Hossler also enter the weekend one stroke off the leaders, with world number one Scottie Scheffler among the five compatriots a further shot behind on three under.
Surprise first-round leader Adam Hadwin dropped to three shots off the front following a second round two-over 72, carding the same score as PGA Tour rookie Callum Tarren, one stroke behind the Canadian on two-under.
With the field cut to the top 60 players and ties heading into the weekend, spare a thought for Cameron Young, who missed the three-under benchmark by a single shot despite a remarkable back nine.
Nine-over par overall after 13 holes following a run of bogey, bogey, par, double bogey, bogey, the 25-year-old had a 0.6% probability of making the cut, according to the US Open website.
Young proceeded to birdie before sinking a hole-in-one – the 48th in US Open history – on the par-three sixth hole.
An ecstatic Brookline crowd had barely had a chance to catch their breath before Young had rattled off back-to-back birdies. Needing to make it three in a row to make the cut, Young fell agonizingly short of a miraculous comeback with par at the final hole.
“It’s hard to sit here and be that pleased that I made a hole-in-one when I missed the cut,” Young told reporters.
World number six Cameron Smith was the highest ranked casualty of the cut, the Australian carding rounds of 72 and 74 to finish six over par.