Collect Signatures for Letters
"Dear Colleague" letters are also often used to solicit other Members to cosign letters to congressional leadership, committee chairs and executive branch officials. Past analysis of "Dear Colleague" letters found that 20.8% of letters in the 111th Congress asked other Members to sign letters.23 In the 113th Congress, the number of letters asking for signatures increased to 25.4%.
Sending letters to executive branch officials or congressional leadership can be an important tool for Members seeking to influence policymaking and gain more support or awareness for a specific topic.24 A letter to congressional leadership, committee chairs, or the executive branch with multiple signers can be used to express Members' opinion on legislation pending before the House or on executive branch policy implementation. A letter signed by multiple Members can also be used in an effort to gain leverage on a policy issue and to demonstrate broad support for a policy position.25
New to the study of "Dear Colleague" letters in the 113th Congress were a limited number of letters that asked Members to sign an Amicus Curiae brief to the Supreme Court. Amicus Curiae are used by individuals or groups who are not directly involved in a lawsuit, but have an interest in or an opinion about the matter.26 In the 111th Congress, five "Dear Colleague" letters mentioned an amicus brief, but none asked for a Member to join as a signing party. By the 113th Congress, this had changed as several "Dear Colleague" letters were sent to ask other Members to sign an amicus brief.27
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https://www.congress.gov/crs-product/R44768
Here is an article that basically shows how it's being done to a degree with the attack on Harvard.
https://www.thecrimson.com/article/2025/4/28/Schumer-letter-april/
When Congress writes these letters (it is one of the things we can use to fight while having very little power). Jamie Raskin talked about this a month or so ago), it puts opinions and official information on the record that may be used in a lawsuit from an outside party.