There are inumerable designs of cross that can be worn as a pin. The simple Latin Cross (shown on the left) is perhaps the most common. Variations include crosses adorned with some other feature, or coloured, or worn in conjunction with a coloured ribbon to Christianise their campaign (see for example the AIDS Cross).
The Lapel Pin Cross, even when used to promote a campaign, is usually not called a Campaign Cross; that term is invariably used for military campaign medals.
People often wear a Lapel Pin Cross or Witness Cross as a symbol of their Christianity. The cross is so distinctive and recognisable, that more often than not, people know the wearer has a message to tell.
And the wearer must realise that their own enthusiasm for Jesus is not shared by everybody. The Lapel Pin Cross, subtle though it is, can appear as in-your-face evangelism and turn off many people who might well convert with a different approach. Christianity is one of those things that people should be able to figure out from the way you live. As St. Francis of Assisi said, "Preach the Gospel of Christ to all; use words if you must."
Having said that, sometimes there's a case for in-your-face evangelism and some people feel they should wear a Lapel Pin Cross while they can; if only in defiance against the current rabid secular campaign to deny religious symbols.
Wearing a Lapel Pin Cross could provoke reactions such as: "So you're a Christian. What's your view on homosexuality?", or to simply be given the cold shoulder. The wearer must also realise that some people will be watching for any slip. Some people just love to catch other people out! Wearers of a Lapel Pin Cross are pretty brave.