Leeds manager Simon Grayson sent on Somma for Sanchez Watt straight after Pompey had regained the lead through John Utaka against the run of play in the 61st minute, and the striker equalised with his first touch.
Portsmouth had made the better start and led 1-0 at half-time thanks to Joel Ward's 26th-minute strike, but the visitors, determined not to let their midweek FA Cup exploits distract them from their promotion push, levelled through Luciano Becchio two minutes into the second half.
Portsmouth fought back from 3-1 down to clinch a point at Elland Road at the end of December, but lost their next three league games and were dumped out of the FA Cup at Brighton, so a point to keep them five clear of the relegation zone was met with relief.
Leeds, for whom Becchio and Leigh Bromby returned to the starting XI, were first to threaten when Max Gradel forced Portsmouth goalkeeper Richard Ashdown to punch away his thumping 25-yard drive in the second minute.
Portsmouth responded and almost took the lead two minutes later when Ibrahima Sonko met Liam Lawrence's corner with a firm header, only for the ball to rebound off the crossbar.
Greg Halford, back in Portsmouth's team for the suspended Aaron Mokoena, twice went close with headers from two more Lawrence corners and both David Nugent and Utaka fired off target as the home side built up a head of steam.
Portsmouth deservedly made the breakthrough in the 26th minute and Lawrence was again the provider.
The former Sunderland and Stoke midfielder weaved his way into the penalty area from the left and pulled the ball back for Ward, who crashed a right-footed shot first time inside the near post.
Leeds goalkeeper Kasper Schmeichel then kept out Utaka's shot from inside the penalty area with a flying save.
The visitors went in at half-time a goal down, but did not have to wait long after the restart to haul themselves level.
Portsmouth defender Carl Dickinson was penalised for handball outside the penalty area, and when Howson's subsequent free-kick ran along the six-yard box Becchio pounced to turn the ball home for his 14th goal of the season.
Portsmouth then regained the lead against the run of play as Nwankwo Kanu headed the ball on into the area and Utaka arrowed an angled shot into the bottom corner.
Leeds sent on Somma for Watt straight after Utaka's goal and a minute later he slammed home a fine goal for the visitors.
Becchio darted into space on the left and moved the ball on quickly for Somma to fire his ninth goal of the campaign into Ashdown's bottom right-hand corner.
The game was temporarily halted due to floodlight failure with quarter of an hour left, but resumed shortly after with Leeds finishing the stronger.
Howson's shot from the edge of the area was turned away at full stretch by Ashdown and Snodgrass pulled his effort wide in the closing stages.
The floodlights failed again with five minutes remaining and neither side could find the winner when it resumed for a second time.