Purist is someone exactly like Mark Levin. To get all Merriam Webster here, it's someone ideological at the expense of being practical, flexible, pragmatic. Needs a candidate to show an exaggerated preponderance of alignment with a person's own dogmatics(His love of Ted Cruz demonstrated this). One thing that can never be compromised is my faith in Jesus Christ, that is immutable and I'm a purist with that as we are told to be. Politicians I give a wider berth
Purism as Mark Levin has demonstrated and spoken about, is a religious like philosophical immutability to broad political slogans and theories. He literally worships at the altar of the mutterings of such 18th-century luminaries as De Tocqueville , Irishman Edmund Burke and some of our Founding Fathers. The Founding Fathers were brilliant no doubt, but unlike the Bible the Declaration of Ind and Constitution has logical holes, contradictions and omissions..as most human originated documents have. No term limits on Congress, no way to effectively and easily overrule the SC, slavery, etc. Being the best system of government ever still is not even in the same universe as a religious faith. It's still trying to perfect humanity, which is impossible. All we can do is mitigate the disaster that is human nature. Sorry Mark, if we could resurrect Reagan and had a super majority of Conservatives, I doubt this country would be near what you think it could be. American gets the government it deserves.
My beef with any Trump disparager who claims to be both a strict constructionist and a Conservative, is that he is ignoring the clearly stated intention of the Constitution in the preamble--and thus not really the "purist," that you describe--and thus is neither logically ideological nor practical & pragmatic.
See Who We Are?.