Riding Mower Won't Start?

I have a Craftsman 917.262960 with a Kohler Command 20hp. It runs great after it starts. It has a new starter and a good battery. I just replaced the solenoid. The problem started a couple of months ago and has progressively worsened. Initially, I had to turn the key several times, hearing only a faint "click", before the engine would turn over and start right up. Then it turned into 15 or 20 times, usually with the faint click, until it finally engaged. More recently, I got it to start by hooking up my charger with 100amp boost. After a couple of tries with the faint click it would fire up and run strong. Last time was 2 weeks ago. I installed new solenoid yesterday. Even with the boost I'm not even getting the "click" now. Any suggestions?