From my book The precinct captain’s guide to political victory:

Letters to the editor are a very effective way of getting out your message. It is one of the most popular sections of a newspaper, so if your letter is published many voters will see it. Even if your letter is not published, it lets the editor know that there is reader support for your candidate and your political party. Editors are human and this sort of feedback will make a difference in what is considered newsworthy.

Short letters are far more likely to be published, so keep it under 500 words. Do NOT send the same letter to multiple publications. They might be published in more than one newspaper, and in the age of the Internet it will become obvious that you are spamming the same letter, and goodbye to your credibility and any future chance of getting a letter published.