In the absence of regular opinion polls (and those are by no means always reliable) historians often look to newspapers. Newspapers are usually relatively easy to get hold of and they are often credited with both reflecting public opinion and helping to shape it. They can appear to be excellent guides to public opinion.
But newspapers have their limitations. They can only speak to their readership, which may be small or unrepresentative of the country as a whole. Most newspapers have a fairly well-established general stance, usually left- or right-wing, though some newspapers reflect a religious viewpoint. People usually read the newspaper that chimes in with their own views and seldom bother glancing at those that don’t. This means that newspapers get good at telling their readers what they know their readers want to hear. This doesn’t mean that newspapers are bad sources, but it does mean that the historian has to decide, “Is this newspaper reflecting public opinion, or is it telling its readers what to think?”
The letters page is usually a good guide to newspaper readers’ real views, although you should always remember that the letters that appear in print are those that the letters editor has chosen. Moreover, the letters themselves only reflect the views of those who feel strongly enough about an issue, and have enough self-confidence, to write to the paper in the first place. How can the historian get at the views of those people whose voices and views never made it into print?