As for those who strive in Us,
We surely guide them to Our paths (Quran)
Any researcher in this subject, if he considers nothing but the truth, will find that the text in support of Ali is very clear, like the following saying by the Messenger of Allah: Whoever considers me his master, then Ali is his master. He said it at the end of the Farewell Pilgrimage, when it was confirmed that Ali would succeed, and many people congratulated him on that, including Abu Bakr and Umar who were among the well-wishers, and who were quoted as having said to the Imam:
"Well done, Ibn Abi Talib, overnight you have become a master of all the believers." 
This text has been agreed on by both Shiites and Sunnis, and in fact I have only referred in this study to some Sunni references. and not to all of them, for they are so many.
If the reader wants more information, he may read "al- Ghadir" by al-Amini (thirteen Volumes) in which the writer classifies the sayings of the Prophet according to the Sunnis.
As for the alleged popular election of Abu Bakr on "The Day of al-Saqifah" and his subsequent acclamation in the mosque; it seems that it was just an allegation without foundation. How could it be by popular agreement when so many people were absent during the acclamation? People like: Ali, al-Abbas, most of the house of Bani Hashim, Usama ibn Zayd, al-Zubayr, Salman al-Farisi, Abu Dharr al-Ghifari, al-Miqdad ibn al-Aswad, Ammar ibn Yasir, Hudhayfa ibn al-Yaman, Khuzayma ibn Thabit, Abu Burayd al-Aslami, al-Bura ibn Azib, Abu Ka'b, Sahl ibn Hanif, Saad ibn Ubada, Qays ibn Saad, Abu Ayyub al-Ansari, Jabir ibn Saad, Khalid ibn Saad, and many others. 
So where was that alleged popular agreement? The absence of Ali alone from the acclamation is sufficient to criticize that meeting because he was the only candidate for the caliphate, nominated by the Messenger of Allah, on the assumption that there was no direct text regarding such a nomination.
The acclamation of Abu Bakr was without consultation, in fact it took the people by surprise, especially when the men in charge of the Muslim affairs were busy preparing for the funeral of the Messenger of Allah. The citizens of al-Medinah were shocked by the death of their Prophet, and then they forced the acclamation  on the people. and even threatened to burn the house of Fatima if those who were absent from the acclamation refused to leave it. So how could we say that the acclamation was implemented through consultation and popular agreement?
Umar ibn al-Khattab himself testified that that acclamation was a mistake - may Allah protect the Muslims from its evil -, and that whoever repeated it should be killed, or he might have said that if someone called for a similar action there would he no acclamation for him or for those who acclaimed him. 
Imam Ali said about that acclamation: "By Allah, Ibn Abi Quhafa has got it! And he knows that my position [regarding the caliphate] is like that of the pole in relation to the millstone! The torrent flows from me, and the bird will never reach me!" 
*Source: Then I Was Guided by Sayyid Muhammad Tijani Smaoui. Ahlul Bayt Digital Islamic Library Project. Available From: http://www.al-islam.org/guided. [Accessed June 19, 2009].
Musnad, Ahmed Hanbal, vol 4 p 281
Siyar al Amin, al Ghazali, p 12
Tadhkirat al Awas, Ibn al Jawzi, p 29
Al Riyadh al Nazarah, al Tabari, vol 2 p 169
al Bidayah wan Nihayah, vol 5 p 212
Tarikh, Ibn Asakir, vol 2 p 50
Tafsir, al Razi, vol 3 p 63
al Hawi lil Fatawi, al Suyuti, vol 1 p 112
Tarikh, al Tabari
Ibn al Athir
Tarikh, Qutaybah, vol 1 p 18
Sahih, Bukhari, vol 4 p 127
Sharh, Muhammad Abduh, vol 1 p 34, Sermon as Shaqshaqiyah
This section contains the following articles in relation to Sunni Caliphate and Shiite Imamate: