Probably George HW Bush in 1988 since he’s the last Republican to really clean up in the suburbs, including northeastern socially liberal suburbs that haven’t gone for any Republican since. Maybe Reagan even moreso though just based on the sheer size of his landslide in 84.
Are we defining “secular” as religious “nones” or people who are unaffiliated? Because that was such a small group as to be irrelevant during Reagan’s 1984 campaign.
Probably 1976 then. Jimmy Carter had lots of appeal among evangelical voters.
