Essendon has kept its AFL finals hopes alive and left Sydney in peril after downing the battling Swans by 43 points at Docklands.
Friday night's 15.19 (109) to 10.6 (66) victory means the Bombers remain outside of the top eight only on percentage, while the fifth-placed Swans could finish the weekend out of the eight if other results go against them.
Sydney has never missed the finals under Longmire's eight-year tenure but faces a tough run home with games against Collingwood, Melbourne, GWS and Hawthorn to come.
The Bombers led throughout the second half and booted four unanswered goals in the final term to post their sixth win from seven games.
Skipper Dyson Heppell (31 disposals) led the way with Devon Smith (17 touches, two goals) and Brendon Goddard also influential.
Held goalless for the first time this year during last week's shock loss to Gold Coast, Swans superstar Lance Franklin had another night to forget.
Franklin was towelled up by best-afield Michael Hurley in the first half and had little impact in the game, despite finishing with two majors.
The 31-year-old suffered a bruised heel during round one and Sydney coach John Longmire said before the game that Franklin had "probably trained for about 20 minutes for the season".
The Bombers went a man down midway through the opening term when backman Matt Dea landed heavily in a marking contest and hit his head on the turf.
Dea left the ground on a stretcher and played no further part in the game.
Sydney had their own injury worries with key defender Aliir Aliir (knee) and midfielder Harry Cunningham (ribs) both requiring treatment during the second quarter.
With AFL chief Gillon McLachlan and football operations manager Steve Hocking keen observers from the bench, the game opened up in an entertaining third term.
Goals to Franklin, Heath Grundy and Ben Ronke kept Sydney in the hunt but Essendon's much-improved ball movement proved the difference, allowing speedsters Adam Saad and Anthony McDonald-Tipungwuti space to create.
Sydney enforcer Zak Jones was reported before the opening bounce, striking Zach Merrett with a crude left hook which is certain to face match review scrutiny.
Jones was booked again in the fourth quarter for a high bump on Kyle Langford, who went to the Essendon rooms during the final minutes.
Smith could also be in trouble for a deliberate trip on Sydney's Luke Parker.