Straightly speaking, EC-135 is not a electronic countermeasurment AC. It is a airborne control and communication relay platform used to ensure US capability to control its nuke in case of ground control was disabled by enemy forces. They are part of US nuclear force even though they don't carry any weapon onboard. For more detail please see “operation looking glass"
And yes they are converted from KC-135A, with the refuelling capabilities retained.