Both Muslim and Christian clerics have seen Umaru Yar'Adua in the past week
The US government wants the head of Nigeria's election commission replaced ahead of new polls due in 2011.
Independent National Election Commission (Inec) chairman Maurice Iwu has been blamed for the flawed elections in 2007.
US Assistant Secretary of State for African Affairs Johnnie Carson said Mr Iwu was incapable of organising a credible election.
Nigeria is the third largest supplier of oil to the US.
The 2007 poll overseen by Mr Iwu was widely criticised for irregularities such as ballot-stuffing and voter intimidation.
The election was won by Umaru Yar'Adua, although legal challenges to the result lasted for months afterwards.
Mr Iwu's current term as Inec chairman ends in June 2010 and the Obama administration is keen that his past record be taken into account when a new chairman is appointed.
The US demands came as a new US-Nigeria Binational Commission was formally inaugurated to improve co-operation between the two countries in areas such as trade, good governance and food security.
President Yar'Adua has not been seen in public since he fell ill in November 2009.
There was speculation that Mr Yar'Adua had in fact died.
However, Muslim and Christian clerics who have visited him over the past week say that, although in poor health, the president is still alive.
"We said the purpose of visiting him was to see if he actually exists," Ibrahim Datti Ahmed, chairman of the Supreme Council for Sharia in Nigeria, told the BBC's Focus on Africa programme last week.
Mr Yar'Adua's duties have been assumed by Vice-President Goodluck Jonathan.
In March, Mr Jonathan dissolved the entire cabinet and made new nominations, most of which were accepted by Nigeria's senate last week.
Mr Jonathan's new cabinet is expected to be formally inaugurated later on Tuesday.