Northampton conceded a last-minute equaliser against Cambridge to miss out on all three points.
But they still stretched their lead to 14 points as results elsewhere went their way, with Oxford United being beaten 1-0 at home by Leyton Orient and Plymouth Argyle losing 2-1 away at Accrington Stanley.
Northampton are on 79 points, Oxford and Plymouth are both on 65.
AFC Wimbledon beat Plymouth to secure League One promotion
Those defeats allowed Bristol Rovers to close the gap on the top three, claiming their fourth consecutive win with a 1-0 win over Mansfield Town to move to 62 points.
Portsmouth might have joined them on the same tally, but they slumped to a 3-0 defeat at Fratton Park against Newport County.
Penalty levels League Two play-off first leg between Portsmouth and Plymouth Argyle
Oxford and Bristol Rovers grab promotion as Accrington Stanley blow it