Casiño said he was grateful for Revilla's support but joining the Lakas-Christian Muslim Democrats (CMD) was "highly unlikely, (as a) matter of principle."
"Ang usapan namin ni Bong (Revilla), sa Cavite lang (Revilla and I only talked about Cavite) because that's his bailiwick," Casiño said in a phone interview Monday.
He said he had not received a formal invitation from the party to be its guest candidate.
Casiño, a former party-list representative of the militant Bayan Muna, is a known critic of the Arroyo administration. Arroyo is chairperson emeritus of Lakas-CMD.
Lakas-CMD recently named Revilla, who is reportedly being groomed to be its standard bearer in 2016, as the party chairman.
"It was made on his (Revilla) personal capacity," Casiño said of Revilla's open endorsement of his senatorial bid at an event in Dasmariñas, Cavite, on Sunday.
Revilla's son Jolo is running for vice governor of the vote-rich province.
Lacking hefty campaign funds, Casiño has resorted to alternative campaign strategies, such as "Takbo ni Teddy, Takbo ng Karaniwang Tao" (Teddy's run is the common people's run) to win votes.
He was in Calamba City, Laguna, on Friday and later that evening at the annual February Fair at the University of the Philippines Los Baños (UPLB), from which he graduated.