Carter would still win in 1976. It's hard to imagine how a popular and respected man like Glenn would harm the ticket in anyway, although an argument could be made that as an experienced Senator with insider connections Mondale helped to reconcile Carter with the party establishment that opposed him in OTL. That said, Carter still wins narrowly and his administration still tanks. The VP is an almost powerless sinecure, so it's not likely that having Glenn in that position would really change how Carter governs. The real difference is 1984 - and whether or not Glenn ends up running like Mondale. Glenn wasn't known to be a very good speaker, whereas a big reason that Mondale narrowly beat Hart was his classic "Where's the beef?" smackdown, so it's quite possible that Glenn would actually lose to Hart and then retire from politics altogether. Either way, whether it's Hart or Glenn who faces Reagan, they probably do much better than Mondale who was hurt by several gaffes about taxes and Reagan's age, but they still lose to the popular incumbent.