Special to The New York Times

HAVANA, Oct. 27- The Cuban rebels said today they had shot down a Cuban Army fighter plane that was bombing the Campechuela District of southern Oriente Province.

The rebel radio said two army planes dropped bombs, one of them an incendiary, on a farmer’s home. One of the planes, an F-47, was destroyed as a result of rebel fire and the pilot [Pablo J. Ors Pina] killed, the radio said.

Representative Manuel Leon Ramirez, a candidate of President Batista’s Progressive Action party seeking re-election in the Nov. 3 election, has withdrawn from the contest and joined the insurgents. Alderman Eduardo Roca of Manzanillo, also a Progressive Action candidate for Congress, accompanied Dr. Leon to Señor Castro’s headquarters.

The Two candidates declared tonight in a broadcast over the rebel radio that the coming elections were the “greatest fraud ever perpetrated in Cuba.” They said they did not want to “form part of a Government that had used repression and torture as a system of Government.”

* [Lt. Ors dropped a defective Cuban Army manufactured 250-pound bomb that exploded upon its release. He was aiming for a C-46 cargo plane abandoned by the rebels at a clandestine strip in Cenaguilla]