Donald Trump promised to build a wall along the US–Mexico border and to make Mexico pay for it, but this seems to violate the principle of ‘no taxation without representation’ on which the United States was founded. Some democratic theorists propose even more radical principles of inclusion, such as that all those affected by or subject to a decision should have a say in it. But even a more moderate principle, requiring that those who pay must be represented, is sufficient to show that Trump's proposed border wall lacks democratic legitimacy.