Cotton's full quote: "As the Founding Fathers said, it was the necessary evil upon which the union was built, but the union was built in a way, as Lincoln said, to put slavery on the course to its ultimate extinction.” https://www.arkansasonline.com/news/2020/jul/26/bill-by-cotton-targets-curriculum-on-slavery/

Structurally, this quote is difficult. On one hand, he is saying that slavery being a "necessary evil" was the Founders' view. But he than says Lincoln union was structured to make slavery ultimately extinct.

He seems to be endorsing Lincoln's statement, so by the structural logic of the sentence he would also seem to be endorsing the Founders' statement. Even if not his intent I don't know how you can from looking at the sentence differentiate the points.

At best he's guilty of sloppiness. At worst ...

Also worth noting: Cotton acknowledges that the union was built on slavery. So why is that wrong to teach in schools?