John Glenn is a good soldier, and that explains much about how liberals got to him and made him one of their crew. If he could be convinced that the liberal-democrat world view is good, noble, and patriotic, he is the type that would go for it and not give it much thought.
The trick is in recruiting and convincing him. If the GOP got to him and did this, he would have been a GOPer.
The types of qualities that make Glen who he is also made him susceptible to the liberal world view, once they got to him. They used his strength against him, making it a weakness.
I don’t agree with the politics he advocates but I cut him quite a bit of slack.