Grasroots workers deserting the party

Mumbai, Nov. 17 -- Monday morning saw intense political speculation after Kalyan Shiv Sena MP Anand Paranjpe visited Maharashtra Navnirman Sena (MNS) chief Raj Thackeray at his Dadar house. Still, he scoffed at suggestions that this was a political visit, saying it was a purely personal call. "I came to invite Raj Thackeray for the wedding of my younger brother Amol. We did not discuss any politics," said Paranjpe. The wedding is scheduled for November 19 at Thane. The strong results drawn by Thackeray in the Lok Sabha and Assembly polls, coupled with the dismal showing of the Sena, have fuelled fears of Sena leaders and grassroots workers deserting the party for the MNS.
I know "grassroots workers" means "ordinary workers" and "deserting" means to leave someone without any help at the time of difficulty, but I cannot understand "grassroots workers deserting the party for the MNS".
I know "MNS" is a party name though.

Source : http://in.news.yahoo.com/32/20091117/1053/tnl-raj-thackeray-gets-invite-for-shilpa.html