Perhaps not so surprisingly, individual bars of soap were once packed for shipping in a crate known as (wait for it) a soapbox. Merchants would unpack the soap for display and discard the box itself in the alley. Periodically, a passerby with public speaking on his mind would pick up the discarded container and use it to elevate himself above the crowd as he held forth on a particular topic of interest, usually religious or political in nature. This practice of standing on an actual box while speaking at length on a controversial topic eventually inspired a more metaphorical use of the term "getting on one's soapbox".
Metaphorically speaking, a soapbox would be any public venue that at least tolerates the exercise of free speech, such as a radio talk show program, a community rant section of a newspaper, an Internet-based discussion forum or a public access cable channel. Whenever a caller or poster on one of these forums decides to go on an extended rant about a highly charged political issue, it could be said that he or she has now gotten up on a soapbox. This isn't necessarily a dialogue between the speaker and audience, but more of a one-sided diatribe from a very opinionated speaker.
There are times when getting up on a political soapbox can be an effective tool in a political activist's arsenal. Some might argue that former vice-president Al Gore's frequent pleas for environmental responsibility would be his particular soapbox, but there have been tangible improvements made as a result of his persistent efforts to raise awareness. Many politicians have their own similar issues, such as President George W. Bush's call for improved homeland security against terrorists. A soapbox issue such as a call for racial or social justice or a repeal of the federal income tax can literally define the career of a politician.
Perhaps the main danger of having such an issue is becoming so passionate and relentless about it that the original message gets lost or loses significance with the target audience. Getting on a soapbox about an important issue might make the speaker feel more empowered, but there is a risk that the person might lose his or credibility by not accepting other viewpoints or demonstrate a willingness for compromise. Anti-war activist Cynthia Sheehan, for example, made the war in Iraq her issue but ultimately became more of a lightning rod for criticism by those who supported the war. Getting on a soapbox can bring an important issue to the public's awareness, but it's also important to know when to step off of it and allow others to do their part to effect real changes.