The only way a Democrat breaks ranks on a deal breaker issue such as Judges, bolton, or S.S. is if they are ready to cut ties completely with the Party. Because the Party WILL cut ties with them. If any in the Dem Party ever considered it, it would be Leiberman or Nelson.
Leiberman wouldn't become a Rep, but he might go Independent. He IS a Liberal, but he could pull off the switch better than Jeffords ever did by being a crucial swing vote. Nelson would become a Republican to win re-election.