Suzanne Breen: Mary Lou's off-script comment is first slip-up since becoming boss
For decades a British withdrawal and establishment of a 32-county socialist republic was the foundation stone of Sinn Fein policy.
But that traditional republican demand increasingly took a back seat with the peace process. A border poll as soon as possible became the party's new cry. It's one increasingly heard since the UK voted to leave the European Union, which is entirely understandable.
The anger that leaving the EU has caused among Catholics who previously weren't bothered about a united Ireland is considerable. The nationalist middle-class seems more sympathetic to that goal than at any time since partition.
If her party is serious about Irish unity, it makes no sense whatsoever for Mary Lou McDonald to say she would prefer not to hold a border poll in the context of a "crash or very hard Brexit" which would be the wrong "climate" for the debate.
That is when the chances of winning a referendum would be greatest. Mary Lou would be expected to shout "Bring it on!", not "Put it off!"
The economic uncertainty facing Northern Ireland and the political chaos created would give a stronger hand than ever before to those campaigning for reunification.
A referendum isn't just won or lost on cold, hard facts. Emotion plays a part, and it seems odd that the Sinn Fein president would wish to wait for political calm to descend first. That is completely in line with Leo Varadkar's position, but then Fine Gael has hardly been chomping at the bit for a united Ireland down through the years.
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Speculation will surround Ms McDonald's speedy U-turn and her new message that Brexit must not delay a vote on Irish unity.
Sinn Fein critics will see the turnaround as the hand of those who still wield enormous power in the party behind the scenes.
But the reality is that Ms McDonald's statement on Monday was out of sync not just with the old guard, but with her deputy Michelle O'Neill. Just three months ago the Sinn Fein vice-president argued that the threshold for a border poll had been met and Theresa May should call one.
That has been the Sinn Fein script for two years. Votes weren't even counted in the 2016 Brexit referendum when Declan Kearney said the decision to leave the EU would have huge ramifications here and his party would keep pressing vigorously for a border poll. Similar sentiments were echoed by Martin McGuinness.
Ms McDonald clearly went off script, for whatever reason, on Monday and then corrected herself. She had voiced her personal view in a party which isn't tolerant of such deviation.
It's her first slip-up in what has been a flawless performance since becoming Sinn Fein president.