Mark Krikorian is polite but forthright in his article: How labeling my organization a hate group shuts down public debate:
Since 2007, the Southern Poverty Law Center has methodically added mainstream organizations critical of current immigration policy to its blacklist of “hate groups,” including the Federation for American Immigration Reform, the Immigration Reform Law Institute and Californians for Population Stabilization, among others. In February, my own organization, the Center for Immigration Studies (CIS), got its turn.
wickedness of the SPLC’s blacklist lies in the fact that it conflates groups that really do preach hatred, such as the Ku Klux Klan and Nation of Islam, with ones that simply do not share the SPLC’s political preferences. The obvious goal is to marginalize the organizations in this second category by bullying reporters into avoiding them, scaring away writers and researchers from working for them, and limiting invitations for them to discuss their work.
Of course, when so many in the mainstream media are also totally partisan, as they are today, it's hard to say which is the bigger tool, they or the SPLC.