You're mistaken.
If I'm not mistaken, Lennon was fairly ambivalent in his politics. Many of the high-caliber lefties wanted him to be more active, but he saw it more as a farce than anything else.
By the time of his death he actually flirted with conservatism. Supposedly, he told his agent in 1980 that Reagan would win and would be America's best president in a very long time.
Other interesting info:
http://www.nationalreview.com/rice/rice200512080829.asp
You're mistaken.
Lennon also supported Ronald Reagan in 1980. Lennon met Reagan on December 9, 1974 when they were both interviewed by Howard Cosell and Frank Gifford during halftime at Monday Night Football. Reagan taught Lennon the rules of American football. Lennon met Jimmy Carter at inaugural gala through a mutual friend, art dealer Sam Green (who I've met a few times), and Lennon was angry that Carter didn't recognize him. A friend of mine, Fred Seaman, who was Lennon's personal assistant at the end of his life, said that Lennon hated Carter and felt that he made America look weak. Mike Tree, Lennon's gardener, said in an online interview that John liked Reagan because he heard that Reagan was into astrology and John said, "He'll be the greatest president of our time."
I've also met Lennon's first wife, Cynthia, and May Pang, the woman John lived with during his 18-month separation from Yoko. May said that John had outgrown radical politics by the time he and Yoko separated in 1973.