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However, these things have a way of changing once voting is done.
I would suggest that Labours stance is an attempt to shore up their vote against losses by tactical voting/people voting for the Greens because they'd go into coalition with Labour.
As you say, once the vote is in, and if it becomes apparent that a coalition is on the cards then I'd expect that to change. Labour+UKIP, say.
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The Lib Dem grandees have all said publicly that there is a huge similarity between Lib Dem policy and Labour. Vince Cable and a few others have even been telling people to vote Labour in some places since just after the election was called.
Lib Dems have also stood down in multiple seats to give other candidates a clear run at the Tories.
If we're judging on deeds rather than words, the Lib Dems have been campaigning for a progressive coalition from the outset. Whereas Labour have 100% been fighting one.
That depends 100% on Labour.
All other parties have acted progressively in various parts of the country, including standing down candidates to support others and campaigning for each other.
Labour is an exception. They've actually been incredibly tribal and have expelled people who have been members of the Labour party for decades for having acted progressively.
In some areas Labour have even expelled their candidates.
It's pretty incredible, but the biggest blocker to a coalition (should the Tories fail to have a majority with them plus DUP) is Labour.