I should add that even if they signed pledges, they cannot be held to them because it would be an extra-constitutional “condition of office.”
so if it is determined that they cannot be held to a pledge their word would be broken should they not abide by their pledge...
as for a recall of a US senator being unconstitutional it may be decided by the US Supreme Court...and then again maybe not.
http://www.politico.com/news/stories/1110/45347.html