Wrexham moved into the promotion race with three great goals and they have games in hand over their rivals.
Southend went close to scoring early on when Kevin Maher's free-kick was headed off the line and they had another chance deep into injury time when Carlos Edwards denied Stephen Kelly with another goal-line clearance.
Unhappily for them they had little to offer in between and were soon chasing the game.
Wrexham opened their account with a brilliant strike. Paul Edwards beat Daryl Sutch and crossed from the left, Damon Searle headed away and Carlos Edwards hammered it back into the net from 20 yards.
Andy Morrell and Lee Trundle should have done better when they broke through United's defence with only goalkeeper Darryl Flavahan to beat, but Morrell, who could have passed to his team-mate in the clear, struck a first-time shot that was easily saved.
The second goal was surrounded in controversy as United's Jay Smith lay injured near the halfway line. United expected a free-kick and hesitated but Wrexham swept forward and Trundle crossed for Morrell to head his 21st league goal of the season.
Morrell headed wide when Trundle put him through again, before Wrexham wrapped it up with another superb goal as Morrell chipped over the top of the overlapping Paul Edwards, who smashed the ball past Flahavan from the edge of the box.
Wrexham goalkeeper Andy Dibble had hardly a save to make, though he had to move quickly to parry a Smith header and keep out a 25-yard shot from Steven Clark.
Wrexham managed Denis Smith said: "It was a big game for us against the in-form side in the division. We had some good performances and took our chances well." A disappointed Southend manager Rob Newman said: "I don't know what was wrong with our lads. Our performance was poor.
"Many of them are playing for contracts, but at this rate they could be out of a job at the end of the season. Our work-rate was not good enough."