That would be minor damage compared to releasing the names and addresses of all our intel sources world wide.
Not so sure. Phillip Agee did a pretty good job of that and we’re still here.
The problem is that the granting of a security clearance is an extremely subjective process. To give the power to a President or Congress to deny a security clearance to any candidate for national office would be to effectively ensure that the party in power would remain in power forever.