Donald completed an opening 68 on Friday morning and then added a second round of 72 to finish level par, one shot behind joint clubhouse leaders Mickelson and Horschel.
Horschel had set the target after a 67 and looked like leading on his own until Mickelson birdied the 18th just seconds after the siren signalled play had been suspended for the day.
That gave the five-time US Open runner-up a round of 72 and meant just two players finished the day under par, with Poulter alongside his Ryder Cup team-mates Rose and Donald on level with four holes to play.
Mickelson had asked the group ahead to stand aside on the 18th fairway to ensure his group could tee off before play was suspended, therefore allowing him the option to complete the round and avoid an early restart on Saturday morning.
"I spoke to Dustin (Johnson) on the 17th and they moved out the way so Keegan (Bradley) could hit a tee shot," the left-hander said.
"It was nice of them and it was a nice way for me to finish. I had fought hard all the way and had let a lot of birdie opportunities slip."
Rose echoed the importance of finishing his round, especially after saving par by getting up and down from 110 yards to complete a superb 69.
"That was huge, it could be worth shots tomorrow," he said.
"Instead of a 7:30am restart and then having to kill seven or eight hours, that lie-in is going to feel very, very good."
Donald's rollercoaster round featured four birdies and six bogeys, four of them coming in succession from the fourth.
At 6,996 yards, Merion was feared to be too short by modern standards and last staged a US Open in 1981, but Donald said: "I think Merion is holding its own, for sure."
Meanwhile, Horschel had remarkably hit all 18 greens in regulation during his round of 67 which equalled the lowest of the week.
"It was a great day," said Horschel, whose only bogey came when he three-putted the 115 yard 13th.
"Four birdies at a US Open, I'll take it. I wish I had a couple more though.
"I didn't know I hit every green until I walked off 18. It's a cool thing, but it's not the first time I've hit all 18 greens. I've done it plenty of times in my career. Obviously it's at a US Open, but I think the softness of the greens helped that."