This script prepopulates select box, state, but not radio button, tobacco.

Code:
<script language="javascript" type="text/javascript">

function prepop()
{
document.long_form.state.value="<?php echo $_SESSION['state']; ?>";
document.long_form.tobacco.value="<?php echo $_SESSION['tobacco']; ?>";
}
window.onload=prepop;

</script>
The value for tobacco, yes or no, shows on the page with view source, so it got to the page, but I can't find how to address a radio button so the value is preselected, and I'm out of ideas.

Any suggestions?