As noted above, Mr. Reagan believed that it was the Democrat party that had changed, not the constitutional conservatives who had formerly been members. If you believe the Democrat party did NOT change, in the century and a half preceding Mr. Reagan's presidency (as your previous post suggests), feel free to document your position.
The Democrat Party was the party of slavery. The Republican Party was against it.
The Constitution as it then existed did not prohibit slavery - in fact, it recognized the legality thereof. If the Republican Party position was that slavery should be unconstitutional, then amending the compact was clearly the only means available. You might review some of the amendments that were proposed in that regard (including one by Mr. Lincoln himself, on December 1, 1862).
The South fought to preserve that system as an economy and split from the Union.
Given that the Constitution nowhere prohibited either slavery or seccession, I must ask - why would the federal government use military force to prevent State seccession (and eventually slavery, via the Emancipation Proclamation)?
The Republican didn't go to war primarily to end slavery although it would eventually. It went to war to preserve the Union and won.
Again, the Constitution nowhere prohibited State seccession.
I'm somewhat reminded of today's political climate. For example, most Americans can legally travel by motor vehicle (the Constitution nowhere prohibits such travel). Let's say you're a government official, and you see your neighbor getting in his car. As he's preparing to hit the road, do you stop him? (You're probably saying, "Why would I, unless he's doing something illegal?")
Let's say he's going to Nevada to donate a kidney - would you stop him? But what if he tells you he's headed to Nevada, to get drunk, and 'party' with as many hookers as he can afford? Or legally purchase a firearm - or pay for an abortion, for his wife or daughter? What do you do? And do you act in your official capacity as a government official, or not?
Again, what he plans to do apparently violates no laws. Are his plans moral? (And is that question even relevant?) Would you shoot up his car, and maybe burn his house down? Or let him hit the road?
A more general question might be: is the federal government bound by law, or by morality?
;^)
Clearly the government, as devised by our Founders, is bound by law.