@ddljohn-- Didn't the Opposition Party have South roots?
If they did, it makes sense for them to remain neutral when it comes to slavery because otherwise they couldn't get support in the South. Being outright pro-slavery wouldn't be good either because then the North wouldn't support them.
I guess the Opposition Party wanted to kill two birds with one stone by appearing open to both sides. But I suppose it backfired and didn't exactly go as planned.
Does anyone know if any prestigious politicians at the time joined the Opposition Party from the Whigs or Republicans?
It would be interesting to know what kind of leadership they had, because that might have been another major reason why the Opposition Party didn't do well.