Everyone thinks it's Article V allowing the Constitution to be amended, but in reality it's Article III. Theoretically, nothing stops a party with a majority in Congress and the White House from abolishing all lower courts, establishing new courts in their place, doubling the size of the Supreme Court, and, in doing so, gaining absolute control over the Judiciary at all levels of government and thus eliminating any opposition to whatever policies they'd like to impose. You want to sue Congress when it grants sweeping powers to the President? The lower courts don't even stay the law. You appeal to the packed Supreme Court? They refuse to hear the case. You complain that the President and Congress have overstayed their term in office? The courts rule that the Constitution does not mandate Federal elections every two years during a state of emergency. You take to the streets and protest? The President declares a state of rebellion and Congress suspends habeas corpus. Any political party sufficiently united in its desire to completely destroy the political system, and having obtained even the slimmest majorities in both the legislative and executive branches, can essentially overthrow any republican system of government. All of this is procedurally allowed. There is no constitutional order, including our own, that can ultimately guarantee its own continuation through constitutional procedures alone.