Your 1st-person narrator can say anything that pops into their head.
Generally, for commonly known facts, you don't need to explain how they know it. If your narrator says that two plus two is four, you don't need to have them attribute that information to their kindergarten teacher. If you're writing in the present day, you could similarly drop a comment that Carter lost in 1980, and you don't have to attribute that information to their history teacher, or the newspapers or television if they're old enough. The reader can just assume that the narrator picked that information up somewhere along the line, and how is not usually particularly relevant.
If how they discovered the information is relevant within the scope of the story, though, you might want to mention it. Like if your story is set on Tuesday, November 4th, 1980, and your narrator is interested in current events, you might mention your narrator discovering Carter's loss. But if contemporary political events aren't really relevant to the story, if your narrator is more interested in sportsball and girls than politics, you might just mention Carter's loss as a background event without going into any detail about it.