A private U.S. conservative group endorsed the candidacy of Corazon Aquino, who is challenging Ferdinand E. Marcos in a Feb. 7 election for the Philippine presidency. The National Defense Council, which has been a prominent supporter of President Reagan's policies in Central America, said, "Mrs. Aquino expresses democratic and anti-Communist views. . . ." The group's decision to back Aquino reflects a split in U.S. conservative ranks over Marcos, who has been praised as a bulwark against communism by such figures as the Rev. Jerry Falwell. Other U.S. conservatives have expressed fear that Aquino might be naive in offering to accept Communist support.