“[N]o Senator or Representative, or Person holding an Office of Trust or Profit under the United States, shall be appointed an Elector” (Art II, Sec 1, Cl 2).
The issue would be if Clinton could be considered in one of these categories listed above, that forbids him from being an elector. If so, his office could be constitutionally challenged.
Being “former president” isn’t covered by those in that list?
Anyhow...I was kind of surprised he was allowed since he does have a very vested interest in the results.