As [link|http://sunsite.berkeley.edu/uchistory/archives_exhibits/loyaltyoath/| here], re Prof. Ernst Cantorowitz and.. years of archaeology concerning '50s McCarthyism and, about the simple fact you state as well:
A coerced oath is invalid on its face.
Only dictators imagine otherwise - or require such for mere daily humiliation.
Anyone remotely literate, with functioning mind and a smattering of knowledge of human history - perceives this, prima facie - but.. none of the above is a prerequisite for holding office amidst non-voting terrified sheep.
A.