82 D50 Electrical Problem

I have an '80 Dodge D50 with the 2.0 liter engine. When I turn on the key, I have power but when I crank I get nothing but a single click. Then everything goes dead. If I remove the negative battery cable and bump it a couple of times on the terminal, I hear it arc and then I can replace it and have power again. But everytime I try to crank the truck it shuts down all the power. The cranking problem has been occuring for about a month now, but only once or twice before the truck starts normally. But it never lost power before today, and never didn't start before today. Any ideas as to what could be the problem?