The Cup is in the Honda Center and the Ducks hope to be sipping from it by the game's end. The Senators are trying to send the series back to Ottawa for Game 6 on Saturday night.
The hero in a 3-2 Game 4 victory when he scored twice and set up the winner, McDonald got the crowd rocking with his 10th of the playoffs at 3:41 and Niedermayer turned up the pandemonium with his fifth at 17:41.
The Ducks were bolstered by the return of defenceman Chris Pronger, who missed Game 4 while serving a one-game suspension for his elbow to Dean McAmmond's head, and Chris Kunitz, back after an abdominal bruise forced him out of the 3-2 win.
The Ducks carried the early play against the nervous-looking Sens, who were hurt by some early undisciplined play.
Defenceman Tom Preissing was called for interference at 1:40 and only a tremendous play by Anton Volchenkov kept Teemu Selanne from putting the puck into an empty net.
Volchenkov, however was called for hooking at 3:25 and the Sens paid for that one.
The Ducks were moving the puck well on the perimeter when it came to McDonald in the right-wing corner. He walked in and his shot deflected off the skate of defenceman Chris Phillips and slipped through Ray Emery's legs at 3:41.
Another Ottawa penalty, this time to Jason Spezza for holding the stick at 5:39, was killed off and momentum began to change soon after.
A big hit on Pronger by Antoine Vermette and Chris Neil helped keep things going and Ducks defenceman Francois Beauchemin made a huge play at the nine-minute mark when he blocked Spezza's shot at an empty net.
Checker Samuel Pahlsson took an elbowing penalty at 10:14 and Ottawa had some great chances but Jean-Sebastien Giguere stood tall.
Things opened after that as the teams traded chances and the Sens had a great opportunity to tie it when Corey Perry took a roughing minor at 15:31.
But again Giguere held down the fort and after the Ducks had killed it, Perry helped clear the puck at the blue-line to spring Niedermayer, who cut in off the wing and put a backhander off Emery and in at 17:41
Selanne took another late penalty but again Ottawa couldn't convert.
The Senators were upset that they went into Game 5 without a morning skate. They asked for 7 a.m. ice time but the Ducks declined, according to Ottawa coach Bryan Murray.
The Ducks had a 9:30 a.m. practice and the Sens could have gone on afterwards, but that would have left insufficient time for his players to properly rest.
Scott and Rob Niedermayer of the Ducks are trying to become the first brothers to win the Stanley Cup together since Brent and Duane Sutter in 1983 with the New York Islanders.