Bath's hopes of snatching a Heineken Cup spot next season are fading fast after Northampton shrugged off the midweek suspensions of skipper Dylan Hartley and Calum Clark to claim a comfortable win at the Recreation Ground.
Saints duly moved back into the play-off places thanks to a first-half penalty try converted by Ryan Lamb, who added two penalties and a drop goal before the break.
Stephen Myler converted a catch-and-drive try credited to former Bath hooker Andy Long to seal the victory.
Stephen Donald kicked two penalties for the home side, whose season is petering out with owner Bruce Craig already seeking a new man to take charge of the playing side from Sir Ian McGeechan.
All Black Donald resumed training only last week after seven weeks out with knee ligament damage, but found himself immediately restored at fly-half as Tom Heathcote was ruled out with whiplash.
Dan Hipkiss made his first start since September, partnering Matt Carraro in the centre, after Olly Barkley was dropped from the squad. Matt Banahan, similarly left out for the trip to Harlequins, returned on the right wing.
Bath saw nothing of the ball in the first 10 and then conceded a five-metre scrum.
Referee Martin Fox awarded Northampton two penalties for collapsing the drive before going under the posts for the penalty try, converted by Lamb.
From Northampton's next attack it took a last-ditch tackle by Carraro to deprive Chris Ashton and the cover was back to shove Christian Day into touch at the flag.
Bath managed to win the line-out but could not escape the Saints stranglehold and the pressure was rewarded by a 24th-minute penalty from Lamb.
It was 30 minutes before Bath finally had a chance of gaining a toehold in the game, with Donald missing a straightforward penalty before succeeding a minute later with a more difficult attempt.
Donald then almost set up a try by gathering his own grubber kick, but Ryan Caldwell could not get his pass away and, with no advantage gained from an offence back at the line-out, Donald kicked a penalty from nearly 50 metres.
However, Northampton finished the half strongly with Lamb first dropping a goal and then knocking over a penalty from wide on the right to make it 16-6.
Caldwell burst away from a ruck on the 22 as the game restarted and forced Ashton to infringe at a ruck, but Donald was wide with the 40-metre penalty.
A minute later, Donald's flat pass put Banahan into space and he kicked ahead for the chasing Carraro.
Ashton got there first but appeared to palm the ball dead. Although the TV match official was able to rule that no Bath player had managed to ground the ball, Ashton escaped sanction.
With Nick Abendanon running dangerously from full-back, it was Saints' turn to feel the pressure, especially when Mark Sorenson was sent to the bin for a cynical infringement.
Abendanon looked as if he would score after rescuing a faltering attack but Ben Foden cut him down just short of the line.
The seven-man Saints pack wilted at a scrum but Donald was wide from 45 metres, his third missed penalty.
When presented with a similar kick, though, Lamb edged his side 19-6 ahead and Bath's cause was further undermined when replacement flanker Guy Mercer was sin-binned on the hour.
Just three minutes later a jubilant Long came up with the ball after the Northampton pack drove over from a line-out and Myler added the conversion to take the lead to an unassailable 20 points.
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