An Atlanta pollster thinks a potentially significant number of people who say they'll vote for Barack Obama won't.

David Johnson raised the issue as he discussed his latest Strategic Vision Poll, which shows the Democratic presidential candidate trailing in Georgia.

Nationally, Mr. Obama is ahead of Republican candidate John McCain by several percentage points in most surveys.

"I think we'll have a Bradley-Wilder effect," Mr. Johnson said Wednesday, referring to two former black Democratic candidates: Tom Bradley, who lost the 1982 race for California governor even though polls just before Election Day said he had a double-digit lead, and Douglas Wilder, who had even bigger leads just before the 1989 governor's election in Virginia but barely won.

Many analysts concluded that some whites were hesitant to declare their support for a white candidate over a black one. Others said some voters let a positive view of a black candidate guide their survey responses but yielded to race-related concerns in the voting booth.

"I think a lot of people aren't ready yet," Mr. Johnson said.

He compared Mr. Obama to 1928 Democratic candidate Al Smith, the first major-party Catholic presidential nominee.

Smith lost but laid the foundation for the 1960 victory of another Catholic, John F. Kennedy, Mr. Johnson said.

Georgia political watchers reacted cautiously to Mr. Johnson's speculation.

"I think he has a valid point, but it's much too early to say how big a one," said Atlanta pollster and political analyst Matt Towery.

He said the impact of the "lie factor" depends on how many people from what groups turn out to vote.

Mr. Obama's strongest support, he said, is from blacks and young people. If as many vote as some predict, the "lie factor" will be diluted, Mr. Towery said.

Not necessarily, said University of Georgia political science professor Charles Bullock. He said an intensely divisive campaign could trigger unusually high turnout among older voters. They are the most likely ones to fib about their preferences, Mr. Towery said.

He said more people will be truthful in polls this year because more surveys use automated phone calls. People are less likely to lie to a machine than a real person, Mr. Towery said.

Mr. Bullock noted that some black candidates do about as well as the polls say they will.

For example, race was widely considered a factor in the 2006 U.S. Senate bid of Harold Ford Jr. of Tennessee. Mr. Ford lost, but his margin of defeat resembled the results of polls taken near Election Day.

Emory University political science professor Merle Black said there might be a "lie factor" this year but he thinks it will be "fairly small" and would matter only in a very close election.

In Mr. Johnson's poll, Mr. McCain led Mr. Obama, 51 percent to 43 percent. Libertarian candidate Bob Barr, a former Georgia GOP congressman, had 3 percent.

"Barr does not seem to be making the inroads among Republicans that he is hoping for," Mr. Johnson said.