Yet it has taken a mere eight days to imbed darkening thoughts that their toppling is imminent.
The blip has arrived, the deep scratch in the polished progress which had characterised half the Blues’ campaign.
It was anticipated, such instances routinely occur, even during highly-successful seasons for clubs, irrespective of their levels.
Successive defeats – coupled with the wretched manner in which they’ve arrived – has amplified doubts. Once they nagged, now they scream.
Admittedly, questioning this Kenny Jackett side is a little harsh, particularly as they continue to set the pace and reside one point clear of the galloping Luton.
The best league start for 57 years, a club record-equalling eight-successive wins and the best away run in Pompey history.
How swiftly such notable achievements slip from the mind.
Ben Thompson has departed, Oli Hawkins and Nathan Thompson sidelined through injury, and suddenly distrust resides in the eyes of the Fratton faithful.
The more hysterical among Pompey’s support are writing off promotion, condemning the current league leaders to a play-off fate – or even worse.
Such is the consequence of two wretched displays culminating in two abject losses over an eight-day period.
Naturally there is a perception among some that the unravelling is underway, the yanking of the thread initiating a process long since feared.
Saturday’s 2-1 reverse at Oxford United was not merely defeat, but the hastening of rapidly-diminishing belief among fans.
The euphoria of Carrow Road was but a fortnight ago. Now a sombre feel has descended, amid supporter demand for the owners to initiate a player outlay, backed by substantial funds.
It remains a testing January both on and off the field of play, replicating 12 months earlier when the same month generated a single point and Checkatrade Trophy elimination.
Still, in the present, Pompey retain a position of immense strength, a placing all League One clubs aspire to possess.
Yet at the Kassam Stadium they once again appeared a shadow of themselves as their hosts, entrenched in a relegation battle, collected a thoroughly-deserved victory.
If Jackett had anticipated a positive response from his players following their lacklustre performance in the 1-0 loss to Blackpool the previous weekend, he was sadly mistaken.
What was served up against Oxford was every bit as dire, arguably marginally worse in some aspects.
Facing a side languishing 21st in the table and nursing a single point from their previous five league fixtures, the match was settled in favour of Karl Robinson’s men by half-time.
Not even substitute Brett Pitman’s stunning goal delivered by overhead kick on 64 minutes could galvanise the limp Blues. It was a moment of quality completely out of step.
Any fear existing among U’s followers at that point was proven unfounded as the visitors barely mustered a meaningful attempt during the remainder of the match. That Pitman brilliance represented nothing more than a lucky punch.
And that is the crux of Jackett’s problems at present. His side are suddenly sapped of spark, devoid of cutting edge and crippled by a plummet in form by a number of previously-reliable performers.
During the last two matches, the team occupying top spot since September 29 have become unrecognisable, coinciding with Ben Thompson's recall, of course.
Pompey’s boss made three changes to his side at the U’s, with Andy Cannon handed a full debut alongside Tom Naylor in the holding-midfield pairing and fit-again Lee Brown returning to left-back.
The final alteration was Pitman dropped to the bench to accommodate David Wheeler’s maiden league start for the club following a patient wait of six months.
There was also a reshuffle within the line-up, Gareth Evans reunited with his role behind the striker following the disastrous outing as a holding player against Blackpool.
Most notable, however, was Jamal Lowe presented with the remit of lone striker, permitting Wheeler to slot into the vacated position on the right of the attacking three.
It represented something of a gamble by Jackett, removing one of his star performers from a wide-right attacking role in which he has consistently been outstanding. Unfortunately, the experiment failed.
Wheeler lasted 58 minutes before withdrawn, although perhaps the mitigation of being denied regular match minutes since his August arrival should be raised before applying warranted criticism over his display.
Nonetheless, the QPR loanee badly struggled in only his fifth start in all competitions for the club, even asked to switch positions with Lowe following the interval.
On the opposite flank, Ronan Curtis is experiencing a lull after those giddy heights accomplished for the majority of the season’s run. He will rise again, of that there is no doubt.
Between them, the starting four occupying attacking roles were ineffective, insipid and uninspiring. Something to worry about considering that issue has now emerged in successive fixtures.
They weren’t alone, however. Cannon was energetic yet too often on the periphery, the usually peerless Matt Clarke – on his 150th Pompey outing – was uncharacteristically shaky, while Anton Walkes was rollicked by several team-mates during the second half.
As a team, there were too many hopeful punts hoisted upfield without Hawkins to retrieve, cheaply surrendering possession and, in the first-half especially, made viewing painful.
Approaching the final whistle – and confident of victory – Oxford supporters even mocked the Blues’ credentials as table toppers. From the evidence before them, they had a point.
Jackett’s men were actually perfectly adequate before the hosts took a 24th-minute lead through Cameron Brannagan. Up until that moment there was a suggestion the Blues could get back to winning ways.
Then Gavin Whyte cut infield from the right, squaring a pass to Brannagan in a central position, who took two touches before rifling a 20-yard right-footed shot into the far corner. He had done so without the arrival of a single challenge.
Suddenly Pompey creaked, losing their composure, and the situation worsened during first-half stoppage time following Jordan Graham’s excellent surge down the left from just past the halfway line.
The winger pulled a cross back from the byline to find Jamie Mackie, who took several touches inside the box before nudging it for James Henry to rifle a right-footed shot high into the net.
Jackett introduced Pitman and Louis Dennis in a double substitution on 58 minutes and the latter soon marked his Football League debut by delivering an assist.
Producing a cross from the right, it picked out Pitman who, eight yards out with back to goal, conjured up a sublime overhead kick to steer it into the net.
But that was it from the Blues, no grandstand reaction, barely an attempt on goal thereafter.
And, after approaching four months heading League One, there is understandably now escalating unease about Pompey staying there.