What was the argest congressional letter writing campaign in US history?

Question

In the United States, there is a system where folks can write letters to their representative and senators in congress to bring attention to a particular issue, sometimes as a timely event or a hot-topic issue that is getting a lot of attention; in an attempt to sway an upcoming congressional vote. What magnitude have been the largest letter writing campaigns in history been? 10,000 letters? 100,000? I'm asking particularly about events where there has been a letter-writing push centered around a particular event - not just gradual annual totals. Particularly interested in physical letter drives. This question is not about petitions or email drives.