In the original On a Clear Day, Barbara Harris played both Daisy Gamble and Melinda Wells -- this dual role was definitely a lead.
In the 2011 revival, Jessie Mueller played Melinda Wells, but David Turner was cast as DAVID Gamble. The role of Melinda in this revised version was probably not quite large enough to be considered a lead, although the character of Melinda was given musical numbers from the Lerner/Lane catalogue to boost her stage presence.