The Alliance Party and the Ulster Unionists both quit a cross-party working group complaining about the lack of progress.
In August, the former Fermanagh South Tyrone MP, Ken Maginnis, quit the party after a row over controversial comments he made about homosexuality.
He told Minnesota party members in a letter outlining the reasons for his decision to quit that the potential for a third major national party still existed.
Attlee was unable to subdue Nye Bevan - a heroic figure for the party's left - who quit the shadow cabinet and was almost expelled from the party a year later.
His implicit threat to quit came at a meeting of party workers as he embarked on a campaign to win support for his plans ahead of a party conference on June 1st.
Both men have quit the Ulster Unionist Party over the decision of the party to support a unionist unity candidate in the Mid-Ulster by-election.
He came close to losing Mr Davis's successor, Theresa May, while the party treasurer, Sir Stanley Kalms, an electrical retailing tycoon, quit a few months later.
The Tea Party drove one long-time Republican operative to quit after 30 years as a Capitol Hill staffer.
She quit the paternal United Party in 1959, frustrated that it was so wobbly against apartheid, to join a Progressive Party of 12 members that was wiped out in an election two years later.
Mr Allawi is a secular-minded member of Iraq's Shia Muslim majority, who quit Saddam's Baath party in the 1970s and, with the backing of American and British intelligence, formed the Iraqi National Accord, which opposed Saddam from exile.
He recalls how Palin quit over the ethics violations of another commissioner, a man who also happened to be the chair of the state Republican Party.
But Mr Trimble will face a crunch meeting of his party's ruling council in January which could put pressure on him to quit the power-sharing Stormont executive if there is no movement on disarmament.
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