The most egregious culprits in this case, the California Republican Party, could have very easily invited him to leave. Davis would attack McC like he did Simon, but Simon lost precisely because he didn't stick to what he claimed were his principles when the pressure mounted (hence his endorsement of Arnold).
Additionally, McC probably wouldn't have a chance had Arnold attended that debate, and provided the publicity for McClintock's competence. Arnold did, so McC does. If Arnold dropped out, McClintock would win. California, as liberal as it supposedly is, would elect a conservative. It is possible. The reality of it is clear.