Not just the protagonist, and not just power. There are a great number of LOTR characters tempted by things they greatly desire. Some resist temptation and remain good. Some fail and turn to evil. Some fail and then attempt to redeem themselves. Even Sauron himself is evil by choice rather than nature (although this is not made clear in LOTR, it's very clear in the Silmarilion).
Precisely - he was a Vala corrupted by Morgoth, who tempted him with power. This theme, and its extension through the spectrum of character clear down to poor Gollum, is one of the main points of Tolkien's moral conflict.
For me this is as much "made-up stuff" as, say, Paradise Lost, and for many of the same reasons. Deep waters here...