Obama has allowed his non-supporters to bring freakin' FIREARMS into the crowds at his rallies. Bush would have his people arrest people for just wearing an anti-war T-shirt or button to one of his rallies.

If a Tea Partier wants to get close to Obama and give him a fake "award" - even one that's insulting - I don't think the Secret Service would knock them down and step on their head.
Obama's opponents aren't kept any further away from him than anyone else in the crowd is.

... And he happens to be the PRESIDENT (just as GWB was), not just someone running for Congress.   There's good reason for security being a bit tighter for the President than for just any candidate for office.

Still, even with the Secret Service the duty is to protect the life and limb of the President (and VP, etc.), not to protect them from harassment or embarrassment by probing questions, insulting T-shirts, "fake awards," or dissenting shouts from the crowd.