Andy DC dug up this 1993 NCDC document, which showed that US winter temperatures had remained essentially constant from 1896 to 1993.
That of course isn’t any use for funding, so they have since altered the data to make it appear to be warming.
The animation below shows how they cooled the past.
They used a pretty generic data tampering algorithm, of simply cooling the past linearly to create a non-existent warming trend.
By doing this, they managed to make the post-1920 cooling trend disappear, which is what political expediency requires.
But it is worse than it seems – since 1993, US winter temperatures have plummeted, while NCDC shows the US continuing to warm.