The Essex club's powerful push for promotion is no more surprising than Millwall's steady slide since sitting at the top of the table three months ago. Whelan has them playing with a pace and imagination that have been conspicuously absent from Millwall during a dismal run of one win in 15 games.
Mark McNally's 34th-minute header and a low drive by David Regis 13 minutes from time made sure of the points for Southend, who would have won by a wider margin but for some superb saves by Tim Carter.
A 1-1 draw for the leaders Derby at Grimsby dragged them back within range of second-placed Charlton, who play today. Grimsby's Italian import Ivano Bonetti opened the scoring with a header from John McDermott's cross, but even though the home side had the better chances after the break, the only second-half goal was Darryl Powell's equaliser from Robin van der Laan's through ball.
Huddersfield Town are third after beating Tranmere Rovers 1-0, substitute Simon Collins ending a sweeping move by heading in Andy Booth's cross to give them their first home win since December.
Sunderland slipped to fifth after a poor first half at Molineux. Wolves, now responding to Mark McGhee's methods, took command with Andy Thompson converting a 15th-minute penalty after a foul on Don Goodman, who scored the second from a rebound five minutes later. Mark Atkins added a third for Wolves on the hour, although by then Sunderland had begun to make a match of it.
Sheffield United reacted to Howard Kendall's decision to put 22 players on the transfer list by beating Oldham 2-1 to move off the bottom of the table. David White no doubt put himself in the manager's good books by scoring both goals.
Partick Thistle are likely to appeal over the sending-off of Rod McDonald early in the second half of their 2-1 home defeat by Rangers. He was shown the red card early in the second half for a foul on John Brown. Partick's manager Murdo MacLeod said: "Rod was yellow-carded as he left the field at the end of the first half for crossing himself. You certainly know which city you are in when you do it here."