"This is the task, I think, of a letter movement. But it should be set up only in states where a significant response can be achieved, for a letter movement necessarily presupposes a strong organization"

Fredrik Bajer is suggesting that a letter movement must be established in states where it can be effective. This suggests that a strong organization is essential for the letter motion to be effective. He is recommending that the job of the letter motion is to create a considerable reaction from individuals in the state. This might be in the type of a petition or a letter writing campaign. He is recommending that the letter motion should be organized in a manner that it can be effective and create an action from the people. He is stressing the significance of having a strong company in order for the letter movement to be effective. He is suggesting that the letter movement need to be established in states where it can be successful and create a reaction from the people.