When Sinn Fein's chairman said Barry McElduff's behaviour was "indefensible", it was clear the West Tyrone MP was under serious pressure.
He faced a mounting chorus of criticism from relatives of the 10 Kingsmill dead, other victims' groups, unionists and nationalists.
Irish Foreign Affairs Minister Simon Coveney attacked his "really, really stupid and insensitive" actions.
Last week, John O'Dowd, a former Sinn Fein minister at Stormont, said the mass killing by republicans in 1976 was shameful and purely sectarian.
The party may have hoped that suspending Mr McElduff for three months would defuse the row.
But the pressure mounted as he faced official complaints to Parliament.
He clung on for a few more days before announcing his resignation on Monday.
The married father-of-three, 51, fell foul of what his own party said was "inexcusable" behaviour after tweeting a video posing with a Kingsmill loaf on the 42nd anniversary of the massacre of the same name.
Ten innocent Protestant workmen were killed in the sectarian shooting by republicans on a rural road in South Armagh.
Mr McElduff apologised and said he did not realise there could be a
possible link between the bread brand and the anniversary.
He once said there should be no hierarchy of victims of the Northern Ireland conflict.
As a schoolboy he took days off to attend the 1981 funerals of IRA hunger strikers.
Sinn Fein's West Tyrone MP has defended the right of republicans to remember their own victims of violence or "patriot dead", including two IRA men killed by their own bomb in his native Co Tyrone.
He added previously: "We all should have the opportunity to remember our dead."
Mr McElduff, who lives in the village of Carrickmore in Co Tyrone, has been steeped in republicanism from a young age and told of his pride at taking the day off school to attend the funeral of IRA hunger striker Raymond McCreesh.
He said: "The level of his commitment and sacrifice is difficult to comprehend."
After a local council controversially named a play park after McCreesh, Mr McElduff said he was a hero.
He said: "There are Irish people in possession of Nobel Prizes for their various contributions.
"As far as I am concerned, Raymond McCreesh would be more deserving of international recognition than many of the past recipients."