There is a petition to allow guns at the Republican convention. When was the last political convention (Democrat or Republican) that allowed people to bring guns?
There is no law saying that guns are barred at conventions. The reason they won't be (weren't) allowed is the Secret Service who reserves the right to restrict firearms under Federal Law 18 U.S. Code 3056. I know this doesn't directly answer the question, but it may help re-frame your research. Unless the Secret Service steps in, whether guns are allowed or not will come down to the specific city's gun laws.