Easiest way to do it is to have not become a Socialist. The options for that are that
-The Supreme Court rules in his favor during In re Debs.
-The Federal Courts that threw him him prison in the first place acquit him
-While in Prison for the Pullman Strike he never starts reading Socialist Literature.
Before he was a convert to the ideology he considered himself a Populist Democrat. As long as men like William Jennings Bryan take over the Party in 1896, and hold onto it after as per OTL (With the exception of handing it over to the Bourbons in 1904) Debs has a decent chance if we get him into some sort of Political Office, like a Congressional Seat, of being an option for 1908 or 1912. He might make really good VP material, and then well its easy enough to do.