Western scholar supports Zaidi Imamate Position:

Wilferd Madelung, a non-Muslim expert in Islamic history, in his book “The Succession to Muhammad” strongly refutes the arguments of the Sunnis that Muhammad (pbuh) was not to be succeeded by any of his family according to God’s design, and that Muhammad (pbuh) wished to leave the succession to be settled by the Muslim community on the basis of consultation (shura). He does this because of the overwhelming evidence he finds in the Qur’an that lineage from Prophets is a prerequisite for legitimate Islamic leadership. He writes in his Introduction:

“The Qur’an accorded the Ahlul Bait of Muhammad an elevated position above the rest of the faithful, similar to the position of the families of the earlier prophets…. It is evident that he could not have considered Abu Bakr his natural successor or have been pleased by his succession…. He could not have seen his succession essentially other than in the light of the narrations of the Qur’an about the succession of the earlier prophets… These earlier prophets considered it a supreme divine favor to be succeeded by their offspring or close kin, for which they implored their Lord…. Even if the meaning of the term “The Seal of the Prophets” is accepted to be the “last of the prophets”, there is no reason it should imply that Muhammad , as the spiritual and worldly leader of the Muslim community, aside from his prophethood, should not be succeeded by his family. In the Qur’an, the descendants and close kin of the prophets are their heirs also in respect to kingship (mulk), rule (hukm), wisdom (hikma), the book and the imamate. The Qur’an advises the faithful to settle some matters by consultation, but not the succession to prophets.” (pp 16-18)

After the introduction, his book goes on to describe the way the leadership was wrongfully handed over to the Prophet’s companions, in great detail, based upon the early source material. But, historical material is fairly subjective, and the best way to judge the historical events is by comparing them with the clear message of the Qur’an, as he has done very thoroughly in his introduction.
To read the complete introduction to the book, and parts of the first chapter, click on this link:

ly+Caliphate&hl=en&ei=kuAzTKHwD5">http://books.google.com.au/books?id=2QKBUwBUWWkC&printsec=frontcover&dq=The+Succession+to+Muhammad:+A+Study+of+the+Early+Caliphate&hl=en&ei=kuAzTKHwD5