How can I programatically activate a GtkRadioToolButton?

[<lindleyf cox net>]

I have a tool bar containing several GtkRadioToolButtons. Clicking on them works fine. However, I'm trying to 
figure out how I can activate one of them programatically.

My first guess was to do
gtk_widget_activate(GTK_WIDGET(radiobutton));
but that didn't work; gtk_widget_activate returned FALSE, and no "clicked" signal was emitted.

I've looked over the other available functions, and nothing strikes me as the obvious choice here. Any 
suggestions?