I needed to do this with inputs, but did not have access to change the html, which was generated by Wordpress and Jquery. Well, maybe I could but it would have been a long workaround. Same reason: accessibility.
So, one possible fix you can use:
#comment-form input[type=text],
#comment-form input[type=email],
#comment-form input[type=url],
#comment-form input[type=password] {
width:40%; margin-right:60%; }
If the error message comes after, it can no longer fit on same line & gets bumped. Of course you'd need to consider whether you can support percentage widths in your layout, but you get the idea.
