It was a day for Danny Lennon's visitors to forget, especially with fellow relegation rivals Kilmarnock winning.
St Mirren handed a debut to new goalkeeper Marian Kello, who had signed on a short-term deal 48 hours earlier - but it was not one to remember for the experienced Slovakian.
United were first to threaten through Ciftci's free-kick which Kello was relieved to see fizz past his right-hand post.
Saints were barely seen as an attacking force until midway through the first half, when John McGinn's delicate through-ball found Stephen Thompson.
Former United ace Thomson took a touch but just as he was about to pull the trigger, United's stand-in captain Gavin Gunning got across to stick out a leg and stop him from shooting.
However, in 28 minutes the Tangerines took a deserved lead.
Rankin's lofted ball over the top of the defence picked out Ciftci, who used his strength to hold off two visiting defenders before keeping his cool to prod a shot past the advancing Kello and into the net.
Six minutes later the Turkish ace was celebrating again as he made it 2-0 to the hosts.
Robertson's deep cross from the left eluded the entire Saints defence, leaving Ciftci free to seize possession.
The former NAC Breda star then knocked the ball past Buddies defender Sean Kelly, who lazily brought him down to leave referee Iain Brines with no option but to point to the spot.
Ciftci got up and made an expert job from the spot as he sent Kello the wrong way from 12 yards.
Saints came out with guns blazing after the break in a bid to throw themselves a lifeline.
But it was United who struck next in 58 minutes to virtually kill the contest.
Ciftci was this time the provider as he rolled a pass to Gauld. The youngster took a touch before unleashing an unstoppable left-foot drive into Kello's top corner.
United were on song by this stage and they almost increased their lead four minutes later.
Robertson's cross fell perfectly for Graham and the big striker's snapshot was instinctively parried by former Hearts star Kello.
Saints were down and out and not even a triple substitution from boss Lennon could change things.
They did have a chance to pull a goal back in 76 minutes but Thompson took the ball too far after rounding home keeper Radoslaw Cierzniak, only for Robertson to intercept at the crucial moment.
But Erskine rounded things off with nine minutes remaining with a fine finish on his right foot, beating Kello from 16 yards after fellow sub Ryan Dow had set him up.
- Sports & Recreation
- Marian Kello
- Nadir Ciftci
- Danny Lennon
- Ryan Gauld