That could go both ways, maybe they didnt own a plantation or have use for a slave. But they still could support it. Or, they could be completely against it, and think its completely wrong and dehumanizing, they could help runaway slaves get to the north.
The correct answer is that international volunteers fought on the Republican side in the Spanish Civil War.
The Republican side was formed around the Government, formed by the Popular Front, which in turn was composed of a coalition of Republican parties - Republican Left and Republican Union - with the Spanish Socialist Workers Party, which had joined the Marxists - Leninists of the Communist Party of Spain and the POUM, the Syndicalist Party of anarchist origin and in Catalonia the left nationalists led by Esquerra Republicana de Catalunya. It was supported by the labor movement and the UGT and CNT unions, which also pursued the social revolution. The Basque Nationalist Party had also opted for the republican side, when the republican courts were about to approve the Statute of Autonomy for the Basque Country.
Although it received hardly any external support from the allied powers of the Second World War, due to the International Non-Intervention Committee, the support of the USSR, which together with Mexico together with France and Poland at the beginning of the contest, stand out; they contributed large quantities of military equipment and advisors, notoriously also the support of what were called the International Brigades.
I would say the answer is anti-federalists
