Footnote: that is, some presidents get buffer zones
As a Footnote to the preceding post, under the Supreme Court decision about the Secret Service agents, it appears that not only are they empowered to just decide that a political opponent of the president is an immediate physical threat, they are free to do this in a grossly politically partisan way:
During the 2009 debate over the health care law, there several confirmed reports of people carrying guns to Obama rallies; in at least two of those cases, the guns were loaded. Although a couple of the people were questioned briefly, there were no reports in any of the news accounts I saw of the Secret Service telling people they had to move "out of weapons range" of the president.
Apparently some types of opponents are more "threatening" than others and loving peace is more "threatening" than loving high-powered weapons.
Sources cited in links: