Gage Larvadain had 104 yards receiving, 88 coming on a scoring connection with backup quarterback Cephus Johnson III, to complete a 20-point fourth quarter that included Kelley's third TD pass and a Justin Dumas 52-yard pick-6.
The Lions led 21-7 at halftime with Kelley running for a 9-yard score then throwing touchdown passes to Jahmon McClendon and Nolan Givan in the first half.
With the win, the Lions (6-1, 4-0) are alone in first place in the Southland Conference after Incarnate Word lost to McNeese 28-20 earlier Saturday.
The Lions came in averaging 50.7 points and 569.5 yards per game this season and finished with 621 yards against the Demons (1-6, 1-3).
Northwestern State scored first on Zachary Clement's 10-yard run but was then held scoreless until there were five minutes left.
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