Review the guidelines carefully--especially when dealing with federal grants--to determine whether you are being asked to submit letters of support or letters of commitment. The trend in recent years is to ask for letters of commitment from project partners or collaborators. Both types of letters are intended to show that other entities favor your project.
Generally a support letter is easier to get than a commitment letter. Why? A support letter may be written by a public official, a community group, a nonprofit, or any number of other entities. This letter merely says that the organization writing the letter supports the project and would like to see it funded.
A commitment letter is written by a project partner or collaborator. It goes beyond merely stating that the group is in favor of the project. It commits to acting as a partner or collaborator by providing resources for the accomplishment of the project. It is important to start early in the grant-writing process to acquire commitment letters. Groups writing commitment letters should be included in the planning and implementation of the project whenever possible.
Be sure to check the RFP carefully to determine whether the funder requires support letters or commitment letters. If commitment letters are requested, but not included, that might be sufficient cause to eliminate the proposal from consideration.