The single engine airplane made a gear up landing that resulted in substantial damage to the airplane. The pilot reported that he was cruising at 10,500 feet, when his autopilot kicked off. He reset the autopilot and it kicked off again a few minutes later. He checked his instruments and found that the ammeter was discharging, with the system voltage rapidly dropping from 28 volts towards 20 volts. He notified ATC that he would descend into a nearby airport, reduced all nonessential electrical loads, and slowed the airplane down. The system voltage was now about 12-14 volts. He setup an orbit over the airport, and attempted lowering the landing gear normally, which was not successful. He made numerous attempts to manually pump down the landing gear using the hand-operated emergency hydraulic pump handle. The nose wheel extended; however, the main landing gear only extended 3 or 4 inches. After circling 30 to 45 minutes, the pilot decided to land the airplane gear up. He landed on the runway, slid about 1,000 feet, and veered off to its south side. Ground crews were able to pull the landing gear into the down and locked position after putting the airplane on jacks. There was no evidence of a hydraulic fluid leak. The pilot reported that during a further examination of the airplane he determined that the landing gear motor malfunctioned, and that he had not properly engaged the emergency pump handle prior to attempting a manual extension of the landing gear.