Although I can't vouch for the specific figures cited, I think this NY Times article has it generally right. Obama, Hillary, and leading Congressional Democrats all say that their single objection to the Colombia trade pact is that Colombia needs to do more to prevent killings of union members. But as the article notes, Colombia has been a violent place all around--murder rates of union members in 2007 were actually way below the murder rate for the population at large. And the murders of union members were only related to union activity in a minority of cases. On top of that, the murder rate has declined dramatially in recent years, and the government has convicted many of the killers of union members.
As I noted in an earlier post, polls show that the trade agreement is overwhelmingly popular in Colombia. I think this is because Colombians recognize that the accord will help lift many of the country's people out of poverty. I think it's almost impossible to imagine that the agreement passes during this election year, but I hope President Obama pushes for its approval.
By the way, as someone with professional and personal connections to Colombia, I often wish I had a good reference to point people to in order to explain what the FARC guerrilla group there is about. I still don't have any suggestions in English, but today Spain's El Pais had this very good article in Spanish, which gives a good description of the FARC, emphasizing the key point, which is that while decades ago the group had populist/leftist ideals, it is now basically a criminal enterprise that funds itself through drug trafficking and kidnapping.