Manchester United closed the gap on leaders Chelsea to 12 points after crushing Portsmouth at Fratton Park.
This was the third double Ronaldo has scored this season
United were three goals ahead by the break, Ruud van Nistelrooy heading in the first after Ryan Giggs hit the bar.
Cristiano Ronaldo then scored twice in seven minutes, first unleashing an unstoppable shot past Dean Kiely and then firing home with a low strike.
Late on Matthew Taylor cleverly headed in Wayne Routledge's diagonal shot, while a Ronaldo free-kick hit the bar.
The defeat left second-from-bottom Portsmouth, who next play leaders Chelsea in the league, eight points away from safety.
The game was over as a contest by half-time as United's ruthless finishing exposed Portsmouth's defensive frailities.
United's opener owed everything to Giggs' ability to take on and beat his man.
He skipped his way past Pedro Mendes, exchanged passes with Van Nistelrooy, before rounding Kiely only to hit the bar with a shot from a tight angle.
But the alert Van Nistelrooy followed up and had the easiest of tasks to head the ball into an open net.
If Portsmouth's defence struggled to deal with the speed and variety of United's play, the home side's attack proved equally impotent.
Wes Brown needed to get in front of Benjamin Mwaruwari and head clear early on, while Lomano LuaLua gave Portsmouth's fans hope with a low shot after shrugging off Rio Ferdinand.
But that was the best Portsmouth could muster in an attacking sense and in that devastating Ronaldo-inspired spell before the interval United took an iron grip of the match.
Ronaldo's first goal came from a shot that was hit with such venom that Kiely had no hope of stopping it.
The Portuguese winger then collected a Wayne Rooney pass, before shooting into the corner of the net, despite Andy O'Brien's desperate efforts to get in a saving tackle.
To their credit Portsmouth worked hard in the second half to ensure they were not further embarrassed.
LuaLua went close with another shot, while Wayne Routledge drilled wide from a promising position.
And on 77 minutes United substitute goalkeeper Howard did well to parry a third LuaLua shot after the Portsmouth striker's persistence saw him wriggle clear.
Howard also had to save a swerving shot from Mendes after a mix-up involving the United keeper and defender Nemanja Vidic.
In the final minutes Pompey's persistence was rewarded when Taylor steered a header past Howard after he craned his neck to get to a Routledge shot.
With Ryan Giggs taken off at half-time United were less threatening in attack, though Rooney went close when he tried to lob Kiely while a superb Ronaldo free-kick skimmed off the bar.
Portsmouth manager Harry Redknapp:
"We can still get out of trouble.
"We did it two years ago when we had 21 points from 28 games and there is still all to play for.
"Every time they had an attack they stuck it in the net, but in the second half we showed great pride and created plenty of chances."
Manchester United Sir Alex Ferguson:
"I think Cristiano Ronaldo is capable of scoring between 15 and 20 goals a season.
"Hopefully that will give him the confidence to go on and score more.
"We hope Chelsea have a bad spell and then who knows?"
Portsmouth: Kiely, Pamarot, O'Brien, Primus, Taylor, O'Neil, Davis (Griffin 55), Pedro Mendes, D'Alessandro (Routledge 45), Mwaruwari, LuaLua.
Subs Not Used: Ashdown, Karadas, Hughes.
Goals: Taylor 87.
Man Utd: Van der Sar (Howard 45), Brown, Ferdinand, Vidic, Silvestre, Ronaldo, Fletcher, Giggs (Smith 45), Park, Rooney (Saha 82), van Nistelrooy.
Subs Not Used: Evra, Richardson.
Goals: van Nistelrooy 18, Ronaldo 38, 45.
Ref: U Rennie (S Yorkshire).