Complex sentence: The last person known to hold this designation was Sen. James O. Eastland, Senator from Mississippi, who was referred to as "Acting Vice President" twice, following the resignation of Vice President Spiro Agnew and the succession of Gerald Ford to the Presidency.
Split into simpler sentences: The last person who could have been considered to be Acting Vice President would have been Sen. James O. Eastland, Senator from Mississippi. Eastland was referred to as "Acting Vice President" twice, following the resignation of Vice President Spiro Agnew and the succession of Gerald Ford to the Presidency.