Ehlers pushed it to 3-0 at 19:30 when he took a pass from Dylan DeMelo at the bottom of the right circle, skated to the high slot and scored on a snap shot inside the left post.
"The power play has been a strength for us all year," Morrissey said. "I don't find myself in the slot too often, but it came right out on my stick and I just had to not miss the net. It was nice to get that second goal, and then the huge third goal at the end of the second period from Ehlers that gave us some more room."
Mercer cut it to 3-1 on a one-timer from the left face-off circle 31 seconds into the third period.
"We're trying to score goals, and we need offense, but when we're going at our best, we play with the process," New Jersey coach Sheldon Keefe said. "I can't tell you how many goals we've scored where one line does the work, building momentum for our team, and they change, and a new line comes on and they reap the rewards. We need lines to build momentum."