[1] Before the election, O'Higgins unsuccessfully proposed that Fine Gael nominate Seán MacBride to challenge the incumbent, Éamon de Valera.

MacBride's involvement with Fine Gael had been strategic, allowing him to advance his policy agenda while maintaining his independence from the party’s core ideology.

By suggesting MacBride as a candidate, O’Higgins and others within Fine Gael aimed to present a challenger who could contend with de Valera on Irish nationalism.

After the MacBride suggestion was declined, O'Higgins encouraged former Taoiseach John A. Costello to enter the race before ultimately being persuaded to stand as Fine Gael's candidate himself.

His father Thomas F. O'Higgins, his brother Michael, his sister-in-law Brigid Hogan-O'Higgins, and his uncle Kevin had all been elected to the Dáil for Fine Gael or its predecessor Cumann na nGaedheal.

In response, Ireland's national radio and television broadcaster RTÉ decided it could not (or would not), cover O'Higgins campaigning as this would be unbalanced in their view.

Fine Gael leader Liam Cosgrove responded by arguing that this was unjust, as although de Valera was not formally campaigning, he regularly appeared on RTÉ radio and television in his capacity as president.

[1] Polling was not yet a feature of Irish politics in 1966; instead parties still generally relied on their constituency branches to provide a sense of grassroots sentiment.

Responding to the momentum of the O'Higgins campaign, Haughey announced £5.5 million in spending targeted at farmers just five days before voting began.

[1] In his autobiography, A Double Life, O'Higgins reflected decades later on his performance in the 1966 presidential race: He believed he had conducted himself honourably and maintained his integrity throughout the contest.