Secret Service spokesman Donovan adds: "We develop security plans and implement them in close cooperation with local law enforcement. And we don't deviate from those plans or change them in midstream because of crowd access issues."
In January, the head of Congress' Homeland Security Committee, Rep. Bennie Thompson, D-Miss., wrote to Homeland Security chief Michael Chertoff to make sure that adequate security was provided for Obama.
"The national and international profile of Sen. Barack Obama gives rise to unique challenges that merit special concern," wrote Thompson. "As an African-American who was witness to some of this nation's most shameful days during the civil rights movement, I know personally that the hatred of some of our fellow citizens can lead to heinous acts of violence.
"I remain concerned that the enormity of the 2008 election provides a terrorist — domestic or foreign — with a potent, high-profile target to make a statement," Thompson added. "We must be better prepared."
In response, Chertoff noted that the Secret Service had provided protection for Obama on 436 trips from May 2007 thru Jan. 11, 2008.
However, increased security can, in turn, create a whole new set of problems. At a rally in Cincinnati, the Secret Service ordered volunteers to block access to camera risers, normally reserved for the media. At a Jersey City event, reporters were held from departing the location because local police would not let media depart until Obama had left the premises.
Despite the complications, Obama seems to have built strong relationships with his security detail. As ABC's Sunlen Miller reports, when Obama has an event in the hometown of a Secret Service staff member, Obama likes to make special mention of the native son. Plus, the Illinois senator often plays basketball with his security guards — he even gathered with them to watch the Super Bowl earlier this month.
Obama's rival for the Democratic nomination, Sen. Hillary Clinton, D-N.Y., has also been guarded by increased security lately. Clinton, as a former first lady, is certainly no stranger to Secret Service protection. But as this campaign has gone on, the security seems to have grown as well.
At her recent campaign stops in Virginia, bomb squads, trained dogs, and wand-wielding security members all protected the New York senator. Reporters traveling with the candidate have also noticed stepped-up safety measures, including additional security sweeps of the campaign bus.
In their dedication to prevent a tragedy involving the politicians, security teams are themselves at risk. Last week in Dallas, Sr. Cpl Victor Lozada-Tirado, a police officer escorting the Clinton campaign motorcade to an event there, died in a motorcycle crash.
"We can't have a rally," Clinton later told an expectant crowd in neighboring Fort Worth, acknowledging the accident. "It would not be appropriate for me to take this opportunity as I had planned to be with you to talk about the election."
Obama said that Lozada-Tirado had previously helped the Obama campaign during their stop in the Dallas area, as well.
"We all have security details whenever we go to a particular city, and Sen. Clinton and I have Secret Service, but it's all done in cooperation with local law enforcement, and oftentimes we get motorcycle details who help set up motorcades and ease traffic when we're traveling into a city and out of a city," Obama said. "It's a reminder of the outstanding work that law enforcement does for us each and every day."
ABC News' David Wright, Sunlen Miller, Jennifer Duck and Eloise Harper contributed to this report.