What EXACTLY do the republican senators have against Dr. Liu's nomination to the Federal court?

They seem to be using vagueries and the usual "legislates from then bench", but this guy has highest academic and legal honors you can get. The bar association rates him EXTREMELY high. I see they are pushing their "vagueries" on FOXNews, but I haven't heard one specific. Can anyone here tell me EXACTLY what is wrong with this guy, besides the fact that he's an Obama nomination?