What has been called the linchpin of the constitution?

Answer 1

Perhaps you are referring to Habeas Corpus. See the following article.

In my opinion it's Amendment X - Powers of the States and People. 'The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.'

Unfortunately, the Supreme Court seems to forget about this amendment most of the time.