Any jQuery gurus out there?

I'm having a minor jQuery issue and was wondering if any of you were experienced with it and could offer some insight.

Basically, I can't use jQuery's .css() function to select an element if I want to apply CSS to that element when the value of the CSS property either is, or contains a variable.

For instance the following function without jQuery works fine...

function leftScroll() {
  if(tOffset < 10) {
    tOffset = tOffset + 10;
    document.getElementById("thumbs").style.marginLeft = tOffset + "px";
    doscroll = setTimeout("leftScroll()", 30);
  }
}

But this one doesn't...

function leftScroll() {
  if(tOffset < 10) {
    tOffset = tOffset + 10;
    $("thumbs").css("marginLeft", tOffset + "px");
    doscroll = setTimeout("leftScroll()", 30);
  }
}

I've also tried passing a hash instead of a key/value pair as per the .css() function's alternate syntax:

$("thumbs").css({"marginLeft: " + tOffset + "px"});

Any thoughts?