Michigan State improved to 5-0 in Big Ten Conference play, moving a half game in front of Michigan for the league lead.
The Spartans held only a four-point lead at halftime in a defensive struggle and the advantage was still just five with 14:19 to play. Michigan State then began to inch further in front and built its first double digit lead with 5:43 remaining on a pair of free throws by Harris.
Northwestern made a mere 4-of-17 from the field in the second half while scoring 18 points.
Harris totaled 14 points for the Spartans (16-1 overall) while Keith Appling had 11. Branden Dawson grabbed 11 rebounds while Michigan State was winning the board battle 38-24.
Tre Demps was the only player to reach double figures for Northwestern (8-10, 1-4 Big Ten), scoring 12 points on 4-of-13 shooting.