LSU-Shreveport (10-0), which led 43-36 at halftime, scored the last six points to win a contest with eight ties and five lead changes.
Jamon Morris, who finished with 14 points and 13 rebounds, hit a basket with 1:36 left to put the Pilots up 75-73. Clarke Jackson stole the ball and hit a layup with 1:02 left to put them ahead of the Demons (4-6) by four points.
A 3-point shot by the Demons' Michael McConathy rattled in and out with 52 seconds left. Jackson got the rebound but Porter missed a 3-point shot with 20 seconds left.
Stevenson sank two free throws after a traveling violation by the Demons with 12 seconds left for the final score.
The Pilots also got 11 points and 12 rebounds from Travis Cash.
Trey Gilder had 31 points and 17 rebounds for the Demons. Gerrell Thomas added 14 points and 13 rebounds, reserve Deividas Petravicius had 13 points, and Colby Bargeman had 10 rebounds as well as six points.
LSUS had earlier exhibition victories over McNeese State and Centenary, Division I rivals of Northwestern.
The Demons were unable to convert a 54-43 rebounding advantage, including 25 offensive rebounds. Gilder was 12-for-22 and Petravicius 4-of-5, but the the rest of the Demons made only 9 of 48 shots from the floor.