Mr Johnson said he did not want to be a distraction for the Obama campaign
An aide to Barack Obama, who was advising the Democratic presidential candidate on his choice of running mate, has resigned from his job.
Jim Johnson left the campaign after reports emerged of below-market rate home loans he is said to have received.
Mr Obama said in a statement that Mr Johnson had left to avoid distracting attention from the selection process.
Meanwhile, Republican John McCain has been criticised for saying it is "not important" when US troops leave Iraq.
Mr McCain said he was in favour of keeping US troops in the country for an indefinite period, as long as they were not sustaining casualties.
Obama supporter Senator John Kerry attacked Mr McCain's comments, saying that for military families "it's the most important thing in the world when [the troops] come home".
The row over Mr McCain's comments came as pressure was building on Mr Johnson over mortgage loans he took out in the 1990s.
He received private loans at below-market rates from major US lender Countrywide soon after he left his position as head of the state-backed lender Fannie Mae, the Wall Street Journal reported.
Mr Obama has been critical of Countrywide and other lenders for their role in the sub-prime loans crisis.
Mr McCain was quick to join in the row over Mr Johnson's loan, arguing that Mr Obama was in no position to criticise mortgage lenders for the sub-prime loans crisis when his adviser was receiving favourable rates from a big lender.
"I think it suggests a bit of a contradiction," he said.
Mr Obama released a statement on Mr Johnson's departure from the campaign.
"Jim did not want to distract in any way from the very important task of gathering information about my vice-presidential nominee," the statement said.
"So he has made a decision to step aside that I accept... I remain grateful to Jim for his service and his efforts in this process."