If Holder's appointment is confirmed as the nation's senior law enforcement officer, he will deal with issues ranging from crime to terrorism.
The source said Democrats in the senate were trying to gauge how much opposition there would be to Holder from Republicans over his role in the pardon of fugitive financier Marc Rich by Clinton seven years ago.
The incident provoked controversy as Rich's ex-wife had been a major donor to Clinton and the Democratic Party and justice department prosecutors were concerned over why Holder had not opposed the pardon, which was issued on Clinton's last day in office.
The senior Democrat said at the issue did not currently appear to be a major obstacle.
Holder acted as a senior legal adviser and helped vet candidates seeking to become Obama's vice-presidential running mate during his presidential campaign.
Like Obama, Holder has said the US must reverse "the disastrous course" set by the administration of George Bush, the US president, by closing the Guantanamo Bay detention centre in Cuba that holds about 250 prisoners.
Holder has also said the US must stop sending prisoners to countries that engage in torture, must end spying activities within the US being executed without a warrant and must be able to declare, without qualification, that the US does not torture people.
The Bush administration, which denies torturing suspects, has been criticised by human rights groups for using waterboarding, a widely condemned technique that simulates drowning, and other extreme interrogation methods on people who have been classed as "terror" suspects.