The Nationals are scrambling to avoid a bruising leadership battle on Monday, which is firming in favour of NSW MP Michael McCormack but may still be turned into a full-blown contest by newcomer David Littleproud, who remains a "dark horse" option in the wake of Barnaby Joyce's sudden resignation.

Campaigning for the top job began immediately after Mr Joyce moved to cauterise weeks of growing anger over his affair with a former staffer, Vikki Campion, who is bearing his child, by announcing on Friday he would quit as Nationals leader and deputy prime minister.