"Legal Question: Does banishing Republican poll watchers shift the Burden of Proof of fraud? That is, normally Republicans would need to prove fraud. However, it seems like Democrats have to prove that there wasn't fraud where they kicked out poll watchers."
Absolutely correct that is legal opinion is supported. It is known as the law of spoilage.
It's spoliation ...
The destruction or alteration of evidence resulting from a party's failure to preserve evidence relevant to a litigation or investigation.
Litigants may face severe sanctions for intentionally destroying paper documents and electronically stored information (ESI) that may be relevant to a lawsuit or government investigation after the party's duty to preserve is triggered. The court may sanction a spoliating party by:
Fining the party.
Issuing a contempt citation.
Permitting an adverse inference jury instruction.
Striking the party's pleadings in whole or in part.
Prohibiting the party from introducing certain evidence.
Dismissing the party's claims or defenses.
(See, for example, Orbit One Commc'ns, Inc. v. Numerex Corp., 271 F.R.D. 429, 435 (S.D.N.Y. 2010); Troup v. Tri-Cty. Confinement Sys., Inc., 708 A.2d 825, 828 (Pa. Super. Ct. 1998).)
The culpability standards for spoliation generally vary by court, except in federal courts FRCP 37(e) governs the culpability standards and sanctions available for ESI spoliation (FRCP 37(e); see also Best Payphones, Inc. v. City of N.Y., , at *3 (E.D.N.Y. Feb. 26, 2016) (recognizing the separate legal analyses governing spoliation of tangible evidence and ESI)).