The latest Gallup survey showed Obama trailing rival Hillary Clinton by 49 percent to 42 percent among Democrats in the contest to select the Democratic nominee to face Republican Sen. John McCain in the November 4 election.
A spokesman for Clinton, a New York senator, said of the security breach, "If it's true, it's reprehensible, and the Bush administration has a responsibility to get to the bottom of it."
A political firestorm erupted in 1992 after State Department officials searched Democratic presidential candidate Bill Clinton's passport and citizenship files. The search coincided with Republican attacks on Clinton for his role in the Vietnam anti-war movement as a student at Oxford University in 1969 and for a trip to Moscow he made at the same time.
An investigation found no laws were violated but that officials exercised poor judgment.
Speaking to reporters, U.S. officials said they had asked the State Department's inspector general to conduct an independent investigation of how and why Obama's records were accessed and what, if anything, was done with the information.
"We are now checking to make sure exactly what information was in the files that were accessed," Kennedy told reporters, saying it would typically include passport applications.
Applicants must provide such sensitive information as a social security number, date of birth, address and telephone number as well as their parents' names and places of birth when they apply for a passport.
The officials said that when a prominent person's passport records are accessed, it triggers an alarm in the computer system and the person who viewed them is questioned to see if there was a legitimate reason for looking at the file.
Despite her lead in the latest poll, Clinton trails Obama in the state-by-state contest for delegates that began in January. The nominees are formally chosen by delegates at the parties' conventions in the summer.
Clinton had hoped to try to chip away at Obama's delegate lead with a rerun of Michigan's contested Democratic presidential primary. But a Clinton-backed "do-over" proposal effectively died in the Michigan Legislature when lawmakers adjourned on Thursday without considering the plan.
Obama opposed rerunning the Michigan primary. The Michigan and Florida Democratic primaries were invalidated because both states ignored party directives and held their balloting earlier than allowed.
