I don't think it's an issue with big donors specifically, or at least not just them. Thing is, a lot of people join ACLU for other causes - abortion is a popular one, for example. And back in 2016, many lefties joined during the Trump scare, on the general basis that something vaguely like ACLU was needed to push back all across the board. Naturally, the people who would do something like that tend to be those who feel strongly about politics and have strong opinions on various issues. But then those new members bring the entirety of their public politics to the internal politics of the organization, not just that one issue they joined over. A kind of Eternal September, if you will.
I don't think there is a solution to this, though. If ACLU doesn't get a steady supply of new members, it will wither and die. If it constrains itself to be small enough that the available supply of new members who fully support the existing platform is sufficient to sustain it (even as that platform becomes further away from the mainstream), it will be too small and lacking power for many important fights.
I don't think there is a solution to this, though. If ACLU doesn't get a steady supply of new members, it will wither and die. If it constrains itself to be small enough that the available supply of new members who fully support the existing platform is sufficient to sustain it (even as that platform becomes further away from the mainstream), it will be too small and lacking power for many important fights.