Campbell stated this in a report titled ‘Nigeria’s 2015 Presidential Election: Contingency Planning Memorandum Update,’ which was published and released by the US Council on Foreign Relations.

A former US Ambassador to Nigeria, John Campbell, has warned that Nigeria is likely to experience large scale violence as a result of the upcoming general elections.
Campbell stated this in a report titled ‘Nigeria’s 2015 Presidential Election: Contingency Planning Memorandum Update,’ which was published and released by the US Council on Foreign Relations.
Campbell also predicts that incumbent, President Goodluck Jonathan is likely to beat strongest opponent, Muhammadu Buhari of the All Progressives Congress (APC) at the polls.

“The upcoming elections are a rematch of the 2011 elections between the incumbent President Goodluck Jonathan (a southern Christian) and Muhammadu Buhari (a northern Muslim and a former military chief). Tension between Washington and Abuja is higher than in 2011, largely over how to respond to the radical Islamist insurgent group, Boko Haram, which is steadily gaining strength in northeast Nigeria,” the report read.
“The APC is stronger than its predecessors (Action Congress of Nigeria and Congress for Progressives Change) and reflects a splintering of the political classes. The government’s inability to defeat Boko Haram, the economic hardships brought on by falling oil prices, and a growing public perception that the Jonathan administration is weak have fueled support for the All Progressives Congress. Though the APC’s voter base is in the North, it enjoys support all over the country, unlike the opposition in 2011,” it continues.
Campbell however notes that Jonathan is at an advantage due to the fact that he is “is at the centre of extensive patronage networks; he has access to the government’s oil revenue; and he and his party largely control the election machinery and ballot-counting infrastructure.”
“It is uncertain whether any provisions will be made for voters in the three northern states placed under a state of emergency because of Boko Haram, as well as the estimated one million people displaced by the insurgency. These displaced voters would likely support Buhari and the APC; their exclusion would benefit Jonathan and the Peoples Democratic Party (PDP). Thus despite the strength of the opposition, Jonathan remains the likely—but not certain—winner,” Campbell stated.
The former Ambassador also warned that the elections could “precipitate violence that could destabilize Nigeria.”
Campbell was ambassador of Nigerian from 2004 to 2007 and in 2010 authored a book about the country titled "Nigeria: Dancing on the Brink".
http://naijafussion..com/2015/02/expect-violence-jonathan-likely-to-win.html

He said Jonathan is the 'likely to win but not the certain winner....

Sai buhari

is this a forewarning from the USA? whatever be the case, we wont be used as instruments of violence. Whoever needs to govern at all cost should start violence or protest with members of his family. Nigerians will not fight with their lives, while the seats of power and wealth is reserved for Tinubu, Jonathan, OBJ and all their children and relatives. Dear Nigerians keep yourself safe at all cost so that we can all benefit from the fruits of democracy as it starts manifesting. When a thief volunteers to help you watch over your house, do not express gratitude but suspicion. To be forewarned is to be forearmed.

Jonathan is at an advantage due to the fact that he is “is at the centre of extensive patronage networks; he has access to the government’s oil revenue; and he and his party largely control the election machinery and ballot-counting infrastructure.”

If this is why you said he may likely win, then I beg to disagree.
The Jonathan I know is on his way out already.