Shivaji was a chieftain in the Pune area in Aurangzeb’s time. Aurangzeb as one can recall, was the last of mogul emperors whom the English defeated to take over the country. For the Wikipedia article click here 
Shivaji was extremely devoted to his mother Jijabai, who was deeply religious. This religious environment had a great impact on Shivaji, and he carefully studied the two great Hindu epics,Ramayana and Mahabharata; these were to influence his lifelong defense of Hindu values.Throughout his life he was deeply interested in religious teachings, and regularly sought the company of Hindu and Sufi saints.
Shivaji was a devout Hindu, but respected all religions within the region. Shivaji had great respect for other contemporary saints, especially Samarth Ramdas, to whom he gave the fort of Parali, later renamed as 'Sajjangad'. Among the various poems written on Shivaji, Ramdas' Shivastuti ("Praise of King Shivaji") is the most famous.Shivaji's son Sambhaji later built a samadhi for Ramdas Swami on Sajjangad upon the latter's death. Samarth Ramdas had also written a letter to Sambhaji guiding him on what to do and what not to do after death of Shivaji.
Shiva sena  as the name stands of a political party suggests that we are still fighting the mughals [Oppressors] or the encroachers of our freedom. When Maharashtra is part of India obviously all of us including Marathi speaking are Indians.
Hence the fight now a days can at best be against the corrupt and corruption.
With this realization, the sena influence can spread beyond Mumbai.
Loves for ones own mother tongue is always admirable indeed. Born in Pune I am in love with Shivaji, Shivaji Nagar, Modern high School where I studied, Pune and Marathi. The Marathi Novel “Khara Udhar” [real Salvation] was one my favorites as a young kid.