Why did southerners believe that they had lost political power in the national government?

In this question, I assume you are referring to the time period around the Civil War. The South believed it was losing political power in the federal government. The South was concerned about the direction events were heading in our country politically. There were more people living in northern states. This gave the North a majority in the House of Representatives. The South was concerned that more free states would join the Union, which would weaken southern influence in the Senate because they would be outnumbered by senators from the free states.

The South was also divided politically. The Democratic Party was divided between the Northern Democrats and the Southern Democrats. The Democrats lost the election of 1860 to Abraham Lincoln and to the Republican Party. The southerners were convinced this would lead to policies that would harm the South. They knew their chances of keeping an equal balance between the free states and the slaves was dwindling. The South was very worried they wouldn’t be able to pass laws that would positively benefit the South.