How can I grab a URL string inside a directory?

Question

I'm using this code to grab Google's utm_campaign from the URL to insert it in a hidden form field:

    <script type="text/javascript">
    function getQueryVariable(variable) {
       var query = window.location.search.substring(1);
       var vars = query.split("&");
       for (var i=0;i<vars.length;i++) {
       var pair = vars[i].split("=");
       if (pair[0] == variable) {
         return pair[1];
       }
     }
   }
   function onLoad() {
      var value = getQueryVariable("utm_campaign");
      var e = document.getElementById('keyword');
      e.value = value;
   }
   </script>

The code works great in the root URL but when I use it inside a subdirectory it stops working.