United recorded their first league victory over Rangers at Tannadice in seven years to all but end the champions' slim chances of retaining their crown.
With a 16-point gap between themselves and Celtic and facing their rivals next week at Parkhead in the cup, the outlook is not good for the Ibrox side.
Despite this, it was the light blues who created the first chance when a dazzling run from Ronald de Boer ended with his venomous shot firing into the back of David McCracken.
Rangers turned up the pressure and almost opened the scoring in the 23rd minute. Mikel Arteta volleyed Nuno Capucho's cross off the back of Derek McInnes, but unfortunately for the Spaniard the ball came back off the inside of the post with Paul Gallacher stranded.
The visitors were made to pay when United scored two goals in three minutes.
Firstly, in the 31st minute when the Tangerines went ahead courtesy of a wonderful move. Having won the ball on the halfway line, Mark Kerr stroked it into the path of Andy McLaren who subsequently sped down the right wing crossing for Jim McIntyre.
The striker headed down for Kerr to follow up with a left-foot rocket, in off the underside of the bar from 14 yards.
Then the Rangers defence were caught sleeping at Barry Robson's corner when the in-swinger deceived them all, allowing McLaren to nod the ball in at the far post from virtually on the goal line.
Astonishingly, approaching the break it was almost game over when McLaren's 15-yard shot was deflected onto the bar.
After substituting the ineffective Arteta and Emerson, the Gers continued to toil throughout the second period - posing little threat to United's defence.
In fact, their performance was summed up by the fact that it was six minutes before the final whistle before Gallacher had to make a save.