Foster's old-style unionism left chasm with McGuinness that could not be bridged
In his three years as Deputy First Minister Martin McGuinness served alongside three DUP first ministers. With one his relationship was astonishingly good, with another it was generally reasonable, while with the third - Arlene Foster - it was frankly terrible.
He got on with Ian Paisley amazingly well, despite the fact that the DUP leader had spent a lifetime fiercely opposing the IRA and Sinn Féin, while McGuinness had built a reputation as a flinty IRA commander.
The two left most of their long-time reputations behind them, replacing old animosities with a new willingness to compromise, both politically and personally. After Paisley's retirement they kept in touch; it is said the two men prayed together, with McGuinness saying on Paisley's death that their friendship existed "until the day he died."
It was obvious that not everyone in the DUP could stomach sharing government with Sinn Féin. But Paisley held it all together until senior party members ganged up on him and told him bluntly that his time was up.
The paradox is that while Paisley forgave McGuinness for everything he had done in his republican past, he made it clear that he never forgave his party colleagues for pushing him out.
Things were a bit rockier with Peter Robinson at the helm, and the McGuinness-Paisley warmth was never re-created. But at the same time they were never really at daggers drawn, and when Robinson became mired in the Iris affair McGuinness and Sinn Féin noticeably refrained from hounding him out of office.
The bottom line with Robinson, according to McGuinness, was that he was a strong supporter of the peace process, adding on a person al note that he regarded him as a friend. Robinson, for example, told McGuinness of his intention to step down well before it was announced publicly.
It is clear that one of McGuinness's personal priorities has been to befriend traditional enemies, and clear too that he had successes in doing so with Paisley and Robinson.
He completely failed however to win over Arlene Foster when she became Northern Ireland's third first minister.
When Paisley and Robinson took over the top job, they abandoned much of their old confrontational style and became, in DUP terms, reformers.
Foster, by contrast, has tended to opt for confrontation rather than reconciliation.
Looking back, Foster may have concluded that was the way to go, coming to believe that something tougher than the Paisley and Robinson attitude was the likeliest way of staying in power, and first and foremost protecting unionist interests.
But the old-style hard line may now have come back to haunt her, together with the heating debacle. McGuinness was most reluctant to call for her head, but there are signs that many in the republican grassroots in effect concluded she was an unreconstructed unionist who was unlikely ever to be a powersharer.
In McGuinness's 10 years as deputy first minister there was no significant Catholic vote for the first two DUP first ministers; but at the same time there were no strong calls along the lines of 'Paisley Must Go' or 'Robinson Must Go.'
McGuinness forged valuable relationships with the first two first ministers but could make no inroads with the third. If Foster had tried a little harder she might have softened her tough image and prevented what may now develop: calls that 'Foster Must Go.'