Why did northerners oppose to admitting another state to the union?

In 1819, the slave holding territory of Missouri applied for admission to the Union. Northern states opposed it, feeling that Southern slave holding states held too much power already. ... This had given the South an advantage in Congress.

Explanation:

during both period congress passed law restricting the civil liberties of american citizen that meet with the overall public support is the one statement that explain the rate scare of the 1920 and 1950 is very similar