Hold the object perpendicular to the floor

How can hold the Middle perpendicular to the floor by moving the mouse ?
I generated a 3d printing support, it has three part : Top, Middle and Bottom which are visible on the following image.
Support added to mouse as helper and by ray-casting the intersect position recognize, support display in the mouse position on the object.
Middle must be exactly perpendicular to the floor by mouse moving. How can I do it?