The "Radical Center" shares a great many ideals and approaches with the "Radical Middle". However, we like to think that the Radical Middle is more conciliatory and ready to bargain, more ready to move from their "radix" ("root") or core positions in order to achieve consensus and political gains. Though it may evolve, the "Radical Center" is not easily moved from its position. It is "radical" in the sense that it is deeply and firmly rooted.
Thus, we might say that the "Radical Center" is more of a philosophy than a political movement or phenomenon, and that the "Radical Middle" is more of a political movement or phenomenon that trends towards the ideals of the "Radical Center".
"Radical Middle" activist, lawyer and writer Mark Satin (wikipedia article) provides a very insightful review of the Radical Center, a seminal book on the philosophy, by Ted Halstead and Michael Lind of the New America Foundation. Mr Satin's review will shed much light on the views and approaches of the Radical Center.