The confirmation of US Treasury Secretary designate Timothy Geithner is reportedly in doubt because of questions about tax records and the immigration status of a former housekeeper.
A spokesman for President-elect Barack Obama issued a statement admitting Mr Geithner had made a "common mistake" on his taxes and was unaware the housekeeper's authorisation had expired during her last three months on the job.
Earlier, the Wall Street Journal reported that senators were probing suggestions that a housekeeper who worked briefly for Mr Geithner did not have proper immigration papers.
The housekeeper was employed by the Geithners from 2004 to 2005.
The Wall Street Journal also said senators were concerned that Mr Geithner did not pay social security and Medicare taxes for "multiple years."
No date has been set yet for mR Geithner's hearing before the Senate finance panel. His nomination needs approval by the full Senate.
In 1993, President Bill Clinton had to withdraw two nominees for attorney general for hiring undocumented immigrants as nannies.