TRANMERE Rovers are in League One action against Shrewsbury Town at Prenton Park this weekend.
Saturday's match, kick off 3pm, will be the 34th league meeting between the two clubs.
Rovers, currently 19th on the table, have won 16 of the previous matches with eight going the way of the 22nd-placed Shrews and nine ending all square.
They are separated by just two points ahead of the fixture and McMahon wants to make the most of home advantage.
Rovers' assistant manager John McMahon is urging fans to back the team at Prenton Park tomorrow.
On their opponents, he said "They're a strong side. We found it tough going to their place earlier in the season.
"They're organised and physical and their goals against record is very good.
"It's a must win game for both sides.
"It's about trying to win every game now.
"Being at home is a bonus. I'd encourage as many fans to get down as possible - it's a cup final.
"The fans can really help, if they can bring a couple of their friends to increase the volume and increase the numbers at the game and really push us and get behind the players."
Meanwhile, McMahon says he has injury doubts over three of his midfielders. "Koumas and Jennings will go to the wire," he added.
"It's difficult to say if they'll play or not.
"Jake Kirby is really struggling, he got a knock similar to Jason Koumas in the reserves on Tuesday, so he's very doubtful."