Actually at that time, there was no fresh influx of immigrants into the South. The North had a blockade around the Atlantic coast.
The Declaration of Independence did not proclaim the existence of one new nation but rather of thirteen independent and sovereign states. If you look it up in the dictionary you will find that the word state is actually synonymous with country. When the Constitution was written each of these states acted in its own sovereign capacity to decide whether or not to join the union. None of them had to.
Indeed, George Washington had already been sworn in as president and the first congress was already in session before Rhode Island decided to join. The Constitution specifically gives some powers to the federal government and explicitly reserves all other powers to the states. Based on all this it could reasonably be argued that the states still possessed the sovereign power to withdraw from a union they had entered voluntarily.
The Southern states put this theory to the test by attempting to secede. The political reason for their secession was to protect the institution of slavery which they saw as threatened by the new Republican party.
Slavery was AN ISSUE, but not THE CAUSE. Slavery would have eventually been abolished for economic reasons without the war. There were some in the North who truly saw slavery as an evil that should be abolished, but the majority was not so vehemently opposed to it. Only a very small percentage in the South owned more than one or two slaves and therefore had an economic interest in maintaining the status quo. Actually, only 5% owned even one slave. Mothers would not send their sons to die to free the slaves or maintain the plantations labor supply. I have read a few Yankee Civil War diaries and NONE said they were fighting to free the slaves. The most common reasons for joining were to preserve the Union and, in Bloody Kansas, revenge for raids by the Missouri pro-slavery forces. I'll admit that I haven't found any Rebel diaries, but I doubt if many said they were fighting to preserve slavery.
With a larger population (20 million vs 5 million citizens plus 4 million slaves), the Northern states controlled the House of Representatives. With a larger number of states (19 vs 11), the North controlled the Senate. The South had an agricultural economy, primarily from exporting cotton. EXPORTS of agricultural products were taxed, keeping prices low. The North had an industrial economy. IMPORTS of finished products were taxed, inflating the value of industrial output. 85% of the US budget was from taxes collected in the Southern states. With the Missouri Compromise and the Kansas-Nebraska Act, the South saw their power in the Federal government being reduced even further. The primary cause of the Civil War was the same as the justification of the American Revolution "Taxation without representation.".