There intention is to make the possibility less popular. In this case, I think THEY made a huge error, because for people like Netanyahu that has been one of the motivations for a two state solution. That, and he knows that long term there can not be a democratic Israel - that is majority Jewish - in all the land.
Kerry actually made the same case to a very conservative Jewish organization - the American Jewish Committee - last spring - after speaking of his history with Israel. It was a very well received speech among much of the Jewish committee. J Street, which is beginning to challenge AIPAC as a Jewish lobbying organization sent the video to all people it had on its list. http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&ved=0CCkQFjAA&url=http%3A%2F%2Fwww.c-spanvideo.org%2Fevent%2F219587&ei=jC7wUpH-E_jMsQSzn4LADA&usg=AFQjCNEs3d7A9DSlqn71jwNPxGUPMC1zwA&sig2=TANKvXUp4iF4yF5nuy6Ebw&bvm=bv.60444564,d.cWc
As Kerry's spokeswoman points out - he is simply speaking of things that others are doing - and that more have said they will join if the peace talks fail. The reason they are not NOW boycotting is that Kerry persuaded them to give the peace talks a chance.
I think all these attacks are actually because Kerry HAS helped both sides focus on the issues and from public comments on actual issues - they are moving together. This does not mean that they will succeed in making peace, but it is a good sign they are still speaking and that the extremists on both sides are concerned.