I doubt it was legal but it was never challenged. I lived through all that and people were just sick of it all. It cost Ford the election and the reason we ended up with the peanut farmer.
It was challenged. Murphy v. Ford
This case, which was dismissed by the District Court, cites as precedent Ex parte Garland 71 U.S. 333. Mr. Garland received from President Grant “a FULL PARDON AND AMNESTY for all offences by him committed, arising from participation, direct or implied, in the said Rebellion, conditioned as follows…” Mr. Garland’s pardon came in the aftermath of the War Between The States. Such conditions certainly were not the case in 1974 when Ford pardoned Nixon nor do those conditions exist now.
The salient difference is crimes committed during war time versus crimes committed during peace time. During war there is a break down of civil authority and general chaos. During the chaos crimes will be committed due to circumstance. During peace there is civil authority and general order. The extraordinary circumstance of war is absent, it is not a mitigating factor which warrants a blanket pardon.
Mr. Garland was a member of “the Congress of the so-called Confederate States from May, 1861, until the final surrender of the forces of such Confederate States — first in the lower house and afterwards in the Senate of that body as the representative of the State of Arkansas, of which he was a citizen”