For Sweetman, this brief period of government would not be repeated, as he would remain in opposition for the rest of his life. Upon the retirement of
Richard Mulcahy as leader of Fine Gael, Sweetman played a significant role in
James Dillon's campaign to become the party's latest leader, fending off both
John A. Costello and
Liam Cosgrave, opposing the former due to his failure to commit to the role full-time and the latter due to antagonism between the two while in government. During the 1960s, Fine Gael itself witnessed a major transformation. This internal revolution culminated in the 'Just Society' document produced by
Declan Costello. The distinctly social democrat flavour of the document was very much at odds with Sweetman's deeply conservative views. However, the support of
Liam Cosgrave and
Garret FitzGerald ensured that the document was adopted as the party's manifesto for the
1965 general election, with Sweetman, Dillon and the rest of the conservative wing capitulating after realising they were outnumbered. Nevertheless, he succeeded in having most of the policy gutted prior to the election. Following Dillon's retirement as leader after his defeat in the
1965 Irish general election, Sweetman aided his former rival, Liam Cosgrave, in becoming Fine Gael's new leader to prevent Declan Costello from becoming a credible challenger. However, the favour was not repaid, with Cosgrave removing Sweetman as Fine Gael's spokesperson on Finance, moving him to Agriculture, having been dissuaded from removing him from the front bench entirely by Dillon and
Tom O'Higgins. While both O'Higgins and FitzGerald exercised considerable influence on the new leader, Sweetman became the dominant influence on Cosgrave following the former's excellent performance as director of elections during the
1966 Irish presidential election. This influence led to Sweetman successfully convincing Cosgrave to oppose
proportional representation during the
1968 Irish constitutional referendums, which put him at odds with the rest of his party who successfully campaigned against the amendment, severely damaging his credibility. In his last election, in June 1969, Sweetman was again returned to the Dáil for a seventh successive term. The
election left only a handful of seats between Fianna Fáil and the opposition. However, Sweetman was far from pleased with this result, looking to replace Cosgrave with a new leader. According to Magill, his plan was to install
Tom Fitzpatrick, a moderate figure acceptable to both Fine Gael's liberal and conservative wings, as an interim leader, until Sweetman's preferred candidate,
John Kelly was elected to the Dáil. ==Death==