Just as they had done in a 3-0 Scottish Cup win over United seven days earlier, Jamie Murphy and Chris Humphrey put Motherwell on their way to victory with first-half strikes.
Johnny Russell headed United back in the game with 11 minutes left but two great late saves from Darren Randolph consigned them to their fourth loss at Fir Park this season - double the number of away defeats they have suffered elsewhere.
The victory moved Motherwell seven points ahead of seventh-placed Inverness, who have two games in hand, to give them the chance to seal a top-half finish on Saturday when they travel to Tynecastle for their last match before the split.
And it helped dispel their tag as an inconsistent side as Stuart McCall enjoyed a third consecutive win for the first time since taking over at the turn of the year.
United had been seeking their seventh successive SPL win and there was nothing to separate the teams in a scrappy opening spell on a poor surface. Russell got the first effort on target after 15 minutes when he turned Stephen Craigan and drove from 25 yards but Randolph comfortably held.
Motherwell were missing the injured Gavin Gunning and Stevie Hammell, who suffered a recurrence of a hamstring strain, and it took them more then 20 minutes to create a chance. Murphy beat Keith Watson before cutting it back to John Sutton, but Scott Severin diverted the shot wide.
Severin, making his first start since a defeat at Fir Park in October, then blocked a powerful Chris Humphrey effort before Randolph held a long-range strike from David Goodwillie.
Murphy soon opened the scoring out of the blue in the 26th minute. The 21-year-old pounced on a loose ball after a Ross Forbes cutback was blocked and burst past two defenders to the byline, where his options appeared to be limited.
But the forward squeezed the ball inside the near post of Dusan Pernis from the tightest of angles to claim his 11th goal of the season.
Recent history repeated itself again in the 41st minute. Humphrey's first touch took him away from Paul Dixon 40 yards out and he ran at the heart of the United defence before evading a challenge and striking a powerful shot that hit Sean Dillon and spun over the head of Pernis.
United came back out fired up and Russell hooked a shot just wide from 16 yards before volleying well off target with Randolph out of his goal after punching clear a long free-kick.
Randolph then made an excellent save when he pushed away Scott Robertson's low drive.
But Motherwell settled down and an uncomfortable looking Pernis palmed away two inswinging free-kicks from Tom Hateley.
United boss Peter Houston made his final switch in the 65th minute when Jon Daly joined earlier substitutes Garry Kenneth and Swanson.
Motherwell were looking comfortable but United got themselves back in the game when Dixon swung in a corner and Russell ran to the near post before glancing a header into the far corner.
United were almost level within 60 seconds when Swanson slipped the ball past Craigan and shot towards the far corner from 16 yards, but Randolph blocked with his legs.
A jaded Motherwell side clung on thanks to a brilliant injury-time save from Randolph, who again blocked with his legs after Swanson had set up Goodwillie 10 yards out.
Source: PA
