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 re-unification. 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. In that same year, the movement became known to the Romulan government & 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 & 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 three defecting members of the Romulan Senate to the USS Enterprise-D. (TNG: "Face of the Enemy")