Why did the North want to block Southern ports during the US Civil War?

The Northern political leadership, namely President Lincoln's Republican Party and the Democrat Party, had not favored the Southern secession. The "War Democrats" represented those people who sought to keep the US unified, even though they had little or no respect for the upstart Republicans that were less than a decade old. They were well aware that had the Democrat Party not split apart in 1860, it became a presidential race among four parties. The Democrats had basically split three ways giving the new Republicans an Excellent chance to win the 1860 presidential elections. They were quite sure that had another Democrat, like the last US President, James Buchanan, had won the White House, the South would have never "revolted". Why President Buchanan did not run for a second term is not clear.With that said, the goal of the Republicans and Democrats of the North had to end the eleven state Southern rebellion. One way to win the 1861 armed conflict was to blockade Southern ports, and by economics, break the back of the rebellion. The South had become a plantation economy and its chief export, cotton, had made the entire US economy wealthy. The US had become by 1860, the largest source of cotton in the world. European nations, especially Great Britain depended on cotton to fuel their textile mills. And, the Northern textile mills were also cotton "customers".