I assume you still want the US to be a country in this scenario. In that case, there are two ways to do this:
1. Avert the Civil War (there was a massive increase in federal power between 1860 and 1865).
2. Make the US bigger. Like, way bigger. With linguistically and culturally different territories spread out so far away from one another, it will have no choice but to be a confederation.
A loose confederation or a federation of sovereign states will probably not stay intact long enough, nor develop the singular vision akin to manifest destiny necessary for it to reach the size of of the OTL USA.