The Romulan underground movement was formed sometime prior to the late 24th century on the planet Romulus by a group of Romulan citizens who opposed the Romulan High Command and who supported a Romulan-Vulcan reunification. Its methods and principles were similar to those of the Cardassian dissident movement which emerged in the Cardassian Union around the same time.
In early-2368, the group was greatly supported when Ambassador Spock traveled to Romulus to join in the re-unification efforts. Spock's personal involvement was criticized initially by Jean-Luc Picard as "cowboy diplomacy" due to the risk and unofficial nature of his efforts. In that same year, the movement became known to the Romulan government and military. Senator Pardek was apparently a member of the movement, but was later revealed to be in league with the Romulan military, which was working to dissolve the movement. (TNG: "Unification I", "Unification II")
Later, in 2369, the movement continued to work toward re-unification. To that end, they abducted Lieutenant Commander Deanna Troi and used her to transport a defecting Romulan vice-proconsul and two aides to the USS Enterprise-D. (TNG: "Face of the Enemy")