The Constitution of the United States at the time specifically allowed and regulated slavery. If we want to go this route, not a single officer should have supported the union.
And at the time, state political leaders believed that participation in the Union was voluntary - that was the whole basis for secession in the first place.
If you're referencing the 3/5th compromise, I'd be hard pressed into believing that's what "regulated" slavery. You're also assuming that the outset of the war was based on slavery, but I also assume you're also going to tell me the larger picture was states' rights as others have mentioned in this thread, in which case, I feel like you're having your cake and eating it too. You can't tell me that it wasn't about slavery, it was about states' rights, but then say "slavery was allowed in the constitution at the time and that's why they should have resigned their commissions!"
I'll agree with your second part that some political leaders believed participation was voluntary. However, and if I had better internet right now, I'd find it, but if I recall, some of the earliest founding fathers like Adams, Madison, and Jefferson (?) had written of a "permanent" union and had previously addressed the idea of secession as being a non-starter once you join the union.
