I give the following link, for those interested, to the often mentioned, as of late, 1973 (Nixon era) DOJ opinion, submitted by Robert Dixon*, that a sitting President
should not be indicted,but rather "that an impeachment proceeding is the only appropriate way to deal with a President while in office." The word "should" is mine, because the opinion does not carry the weight of a formal adjudication. Also, please note, in the Dixon Opinion, the absence of any pronouncement such as "the president has absolute immunity from indictment while in office."
In my opinion, the strongest argument in favor of a sitting president being unindictable was missed by Dixon, i.e., the risk of abusive indictment for political reasons. Even such strong political argument, if formally launched, could be responded to with equally strong counterargument. My overall impression is that the often stated opinion "that a sitting president
can not be indicted," although certainly not without merit so long as the word "should" replaces "can," hangs by a mere thread.
https://fas.org/irp/agency/doj/olc/092473.pdf
(I believe there exists one other argument drafted later, perhaps by Kavanaugh, to similar effect as that of Dixon's. I did not bother to search for it, but certainly Kavenaugh's paper, published in the Minnesota Law Review, in which he put forth his, in my opinion, "screwball" Unitary Executive Theory, necessarily incorporates a similar viewpoint on the indictability of a sitting President.)
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*(edit) I just wanted to add that the Dixon opinion seems to me to be very well researched and competently argued.