WASHINGTON — A man has been arrested for illegal possession of a knife outside Senator Obama’s hotel amid growing fears about threats to kill the man hoping to become America’s first black president. Davit Zakaryan, 24, was questioned and searched after Mr. Obama’s Secret Service guards saw him allegedly loitering outside the Fairfield Inn in Ottumwa, Iowa, just before the Democratic candidate was about to start a day’s campaigning.

It is believed the Secret Service agents became suspicious because they recalled seeing Mr. Zakaryan’s car, with Ohio plates, at an Obama event the previous day.

He has been charged with possession of an illegal weapon and driving without a license.

One source said Mr. Zakaryan, from Cincinnati, gave contradictory answers about why he was outside the hotel and mentioned that he was planning to go to an Obama campaign event.

The 8-inch knife was found in his car. Although police said they had no reason to believe Mr. Zakaryan presented any threat to Mr. Obama, the incident highlighted concerns about the Illinois senator’s safety.

He was given Secret Service protection in May, much earlier than most presidential hopefuls.

With Mr. Obama recording record-breaking fund-raising totals and mounting an increasingly potent threat to Senator Clinton, the Democratic front-runner, winning the race to the White House is becoming a realistic prospect. Assassination is a perennial fear for American politicians, and Mr. Obama’s race and the Muslim background of his father make him a potential target for extremists. On the campaign trail, Mr. Obama’s rhetoric often draws comparisons with President Kennedy and his brother Robert Kennedy, who were shot dead in 1963 and 1968 respectively.

Another of the Illinois senator’s political heroes, President Lincoln, was assassinated in 1865. Earlier this year, Mr. Obama’s wife, Michelle, spoke of her fears that he could be murdered but said, “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.”

While campaigning in Iowa this week, Mr. Obama has been shadowed by at least six Secret Service agents. At an event in a gymnasium in Waterloo, the agents guarded each door and asked anyone approaching Mr. Obama with hands in their pockets to take them out. There have been some threats to Mr. Obama posted on the Internet and incendiary comments about his middle name, Hussein, and his father’s religion.

A taxi driver taking journalists to a Clinton campaign event in Iowa this week remarked that Mr. Obama’s head “would be in a noose within three weeks” if elected president.

Jeff Lord, of the Web site QubeTV, a right-wing site that airs videos critical of Islam, told the Politico Web site recently that he had removed a video, which had been posted four times, because of its objectionable content.

Mr. Lord said, “It featured still pictures about Barack Obama, and it had a picture of him. They had a target drawn on his head, and the F-word and the N-word, saying he needs to be assassinated. Finally, I e-mailed [the video’s creator] that it’s a federal offense to threaten the life of a United States senator.”