So Trump is issuing an executive order to ban Act Blue. The five justices pictured above all stated that contributions by PACs and Corporations could not be restricted because it violates the first amendment. In other words they said that spending money to promote a candidate was protected.
Using that argument, and IANAL, it would appear that banning Act Blue would be illegal based on the ruling in Citizens United.
Now, that ruling, was an abomination and needs to be overturned, but while it is in effect it has to apply to all organizations, PACS and corporations, not just those that support Republican candidates. I hope this makes it all the way to the court so we can watch the right wing court trying to contort themselves to come up with a ruling that backs Trump’s goal without contradicting Citizens United.