Orange County, Calif., home of some of President Reagan's most loyal supporters and launching pad for his 1984 campaign, has not been given up for lost by the Democrats after all.

The Democratic Foundation of Orange County, founded by wealthy developer David Stein "to keep the party flame alive in Orange County," is fighting the county's conservative Republican trend. Democratic registration dropped from 45.5 percent in 1978 to 36.5 percent in 1985, with Republican registration rising to 52.8 percent. Armed with money and computers, "like the Republicans," the group has budgeted $80,000 for a drive to register 17,000 new Democratic voters, mainly in conservative Rep. Robert K. Dornan's (R) district.

The foundation, which operates as a political action committee, also recruits and funds candidates for office in the county. The members are backing Orange County Superior Court Judge David Carter, who is challenging Dornan.

The foundation's reach may be extending. Stein and his group have caught the attention of a string of national Democrats who have paid visits, including Sen. Gary Hart (Colo.), Rep. Richard A. Gephardt (Mo.), former governor Charles S. Robb of Virginia, Gov. Bruce Babbitt of Arizona and National Party Chairman Paul G. Kirk Jr.