No.  Assuming you are a public charity and not a private foundation, you are permitted to lobby on pending legislation so long as it is not a “substantial” part of your activities.  (See Ready Reference Page: “Lobbying Rules Create Opportunity for Charities.”) It is hard to imagine how merely signing on to a list of supporters would be deemed a “substantial” activity.  Even if it is a significant activity, you could elect to measure your lobbying expenses under Section 501(h) and spend up to 20% of your first $500,000 of expenditures on lobbying.  (See Ready Reference Page: “Should Your Organization Elect Under 501(h)?”)

You sound like a perfect example of our adage that “if a charity isn’t lobbying, it probably isn’t doing its job.”  If you care about the rights of persons with disabilities, there is no more important point for leverage and impact than affecting the rights set forth in the ADA.  You could educate the general public for years without having a tiny fraction of the impact you can have by obtaining a few key provisions in the law.

You may want to do more than add your name to a list.  You may want to mobilize your constituents and actively join the lobbying efforts to push what you think is important. You can be sure that there will be vocal opponents to the positions you support. Legislators sometimes listen, but they cannot respond to what they do not hear.