Final answer:
The Democratic Party's most effective means of overthrowing Republican governments in the South was through violence and intimidation tactics carried out by White supremacist terror organizations such as the Ku Klux Klan, the Redshirts, and the White League.
Step-by-step explanation:
The Democratic Party's most effective means of overthrowing Republican governments in the South was by threatening, beating, and killing those who disagreed with them. The Democratic Party in the South relied on White supremacist terror organizations such as the Ku Klux Klan, the Redshirts, and the White League to intimidate Black people and Republicans. These organizations used violence, assault, and intimidation tactics to suppress the Black vote and maintain political predominance in the South.