Rabbi Linzer, I was asked if a kohen married to a גרושה, who is otherwise 100 percent shomer mitzvot, is eligible to be an eid for a wedding? Thank you.
He is pasul mide’orayta. See Shulchan Arukh CM 34:2, which rules that anyone who is liable malkut is pasul for eidut. Rambam (Hilkhot Sanhedrin 19:4, no. 151) explains that a kohen who marries a gerushah is liable malkut. Also see Shulchan Arukh CM 34:23 that rules he is pasul even without a public announcement. Although it sounds like you need eidut to invalidate him, this is a case of ודאי and אנן סהדי. Shulchan Arukh does limit the pasul to cases where it is known to be a sin (CM 34:24), but that certainly applies here.