Another writer pointed out that one of the dangers (of sharing quotes) is, because we like C.S. Lewis (or any other writer), we like to think that he would have said what we want him to say. If we are circulating false quotes, we might be trying to remake him, and his ideas, into something that agrees with our own.
If you are so motivated, you can test if this quote is in The Screwtape Letters using Google Books. Go to The Screwtape Letters page and search for ‘fixated on politics’ inside the book. Unfortunately, if you just search Google Books in general for ‘fixated on politics’ The Screwtape Letters is the first result, not because that quote is in the book, but because the algorithm has learned that this is the result everyone is looking for when they search for that phrase.
However, C. S. Lewis DID write about politics, and he speaks to the ways Christians can get caught up in the fray of our current culture. Notably, this writing was decades ago, and The Screwtape Letters was directed toward the people of England in 1942.
Here is Uncle Screwtape (a representation of Satan) giving advice on how to distract Christians: “Let him begin by treating Patriotism as a part of his religion. Then let him, under the influence of partisan spirit, come to regard it as the most important part.
Then quietly and gradually nurse him on to the stage at which the religion becomes merely part of the ’cause’, in which Christianity is valued chiefly because of the excellent argument it can produce in favor of the… war-effort… Once you have made the World an end, and faith a means, you have almost won your man, and it makes very little difference what kind of worldly end he is pursuing. Provided that meetings, pamphlets, policies, movements, causes, and crusades, matter more to him than prayers and sacraments and charity, he is ours–and the more ‘religious’ (on those terms) the more securely ours.”
Once again, America has an intense presidential election on the horizon. I encourage you to avoid “getting fixated” on politics and the “faults of people they have never met.” Admittedly, this will be difficult for me, so I have to practice what I preach. In doing so, I am reminded by the timeless advice of Rupertus Meldenius (although this wisdom is sometimes credited to St. Augustine): "In essentials, unity; in non-essentials, liberty; in all things, charity."