Many in the trade union movement - especially those with links to Labour - have been wary of a cap on domestic donations in case a future government used this to restrict their donations.
But behind the scenes officials recognise the eye-watering donations from Reform UK have changed the game and some form of domestic cap may have to be considered.
The leader of the Unite union Sharon Graham told the BBC she favoured a cap of £500,000 to £1m on individual donations from UK-based citizens.
But she argued that as union donations are made of lots of individual members making a contribution through a political levy then this should be treated differently.
She said: "There is a difference between a crypto-billionaire based in the UK or abroad giving millions and trade union members who are nurses, cleaners, bus drivers giving 10p a week which adds up cumulatively to what we give to Labour."
The GMB union has been one of the most wary over the implications of a domestic donations cap.
Its general secretary Gary Smith told the BBC: "Whether there will be a cap on political donations, we will wait and see.
"We will talk to the government at the appropriate time but we can't have any confusion between the political voice working people have through their unions and crypto-billionaires."
What is interesting about Tuesday's union statement is not just what it says but what it doesn't.
There has been no consensus on calling for caps on the spending of political parties rather than donations - one option for dealing with the Reform war chest.
Another option was to treat big unions donations as a series of donations from individual members. For example a million strong union that donates £1m to Labour would have been regarded as making donations of £1 a member - thus escaping a cap. Some believe this would be politically difficult to argue.
Not all unions donate to Labour and those which do not are therefore very relaxed about a cap.
One even said the events of the past few days have been helpful in arguing for more restrictions on political donations.
But I understand discussions will continue amongst those unions with financial links to Labour - including the three biggest in the UK - to try to reach a common position on Wednesday behind closed doors in Brighton and possibly in days and weeks ahead.

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