Move when pressed but only within confines of a path

i have no idea how to implement it so that when pressed, the object goes to the mouse position but also only stays on the path(think if it like a slider). i have an iTween path all set up and ready to go but am struggling trying to find a way to implement it.

i have tried

   iTween.MoveTo (gameObject, iTween.Hash ("path", iTweenPath.GetPath ("Path1"), "time", 1));

and when not pressed it pauses and when pressed it resumes. but the problem is that only goes one way and it doesnt move freely. Any help would be appreciated.