As Obama spoke to about 200 people in an elementary school gymnasium, there were at times six Secret Service agents who lined the room, covering every door. If someone approached with his hands in his pockets after the speech, he was quietly asked to show them.

It was the Democratic presidential candidate's first trip to Iowa or New Hampshire — where campaigning in intimate settings is viewed as especially important — since the Secret Service assigned a team Thursday for his security. It is the earliest a candidate, with the exception of Sen. Hillary Rodham Clinton, D-N.Y., has received such protection.

But Obama, who previously traveled with a smaller, private security detail, downplayed any change when asked how his new security bubble would make campaigning different.

"I'm very appreciative of their efforts," the Illinois senator said. "They are obviously extraordinary professionals."

His low-key campaigning in Waterloo, at two events that were not publicized much in advance, took place in a community that has the largest proportion of African-Americans among this state's largest cities.

During the school event and later at a Baptist church where Obama spoke, the agents were visible, but they hung back a bit from the candidate. Still, security is fresh on the minds of Obama and other presidential candidates.

A Louisiana State University student was arrested late last week after he told another student he was planning a violent attack against Clinton.

While Clinton has been under Secret Service protection for years because of her status as a former first lady, the change in Obama's security left former North Carolina Sen. John Edwards as the only top Democratic candidate who maintains his complete freedom of movement.

Edwards said late last week that he has no plans to request Secret Service protection.

"I personally would rather, as long as possible, have the freedom to be able to be with people, to not have barriers between me and them," he told CNN.

That flexibility can be a real advantage in states such as Iowa and New Hampshire where voters have long placed a premium on close interaction with candidates.

The Secret Service detail that surrounds Obama will inevitably create some sense of distance, just as it did for then-Vice President Al Gore when he campaigned in Iowa in 1999 and 2000 with a much larger security detail.

While there are disadvantages to Secret Service protection, there are also significant upsides. It can help smooth some logistics, keep the campaign on time and provide a presidential aura.

All of the security carries a cost beyond the physical limitations placed on the candidate.

The Secret Service bill for protecting presidential candidates this election cycle is expected to cost about $107 million, up significantly from the roughly $73 million spent in the 2004 election cycle.

The Secret Service's director recently said he expects to spend $85.2 million in 2008 to protect presidential candidates. That's on top of this year's $21.4 million.

The increasing cost is partly a result of a wide-open race where for the first time since 1952 neither the president nor vice president is a candidate.

The tradition of protecting major-party candidates started after New York Sen. Robert Kennedy was gunned down in June 1968 after his win in the California primary.

Security has been a serious concern for Obama's wife, Michelle, especially since he started to attract large crowds following his 2004 Democratic National Convention speech.

Still, she has downplayed her worries in recent months. Asked earlier this year by a "60 Minutes" interviewer whether she feared for her husband's life, she replied, "I don't lose sleep over it because the realities are that, as a black man, Barack can get shot going to the gas station."

Sen. Dick Durbin, D-Ill., has said Obama's need for a Secret Service detail has much to do with his race. And his race was an integral part of his Sunday campaigning.

As he travels the nation in a quest to become its first black president, Obama is aggressively wooing African-Americans, a vital constituency for his party.

In the early primary state of South Carolina, blacks account for as much as half of the electorate. And even in Iowa, a state that is mostly white, Obama is targeting potential black caucus participants.

Sunday was his second visit to Waterloo since his February presidential announcement. And he promised Sunday to return again and again.

Iowa is 92 percent non-Hispanic white and Waterloo's African-American population totals about 8,500. The group represents 12.4 percent of the city's population, according to census data, slightly higher than the national average.

Obama started his day by meeting privately with about 75 mostly African-American ministers and pastors from the area.