Raycast Check in GetKey?

I've been developing a character controller from scratch and have manged to get the ground checking working, however when I go to step off a ledge my character continues to float in the direction of travel, and slowly falls downward, instead of plummeting. I think the issues lies in the fact that I can't check for raycast hit while moving and the key is down.

My code for raycasting and checking for the ground is as follows:

 RaycastHit2D hit = Physics2D.Raycast(transform.position, Vector2.down, 0.55f);
 Debug.DrawLine(transform.position,new Vector2(transform.position.x,transform.position.y-0.55f),Color.red);
 
 if (hit) {
     if (hit.collider.tag == "ground"){
         isJumping = false;
     }
 
     if (hit.collider.tag == null){
         Debug.Log ("There is no ground!");
         isJumping = true;
     }
 }

My code for moving left is as follows (rb is my rigidbody):

 if (Input.GetKey (KeyCode.A) && isJumping == false) {
     rb.MovePosition(rb.position + -horizontalVelocity * Time.fixedDeltaTime);
 }

Any help would be appreciated!