What prevents a democracy from becoming an autocracy?

Nope. It's not uncommon for educated people or those with common sense to be targeted by an autocrat after he gained power.

Nope. Demagogues, extremists and autocrats are very common in all societies.

Your answers are interesting. Not correct though.

"By getting money out of politics."
Money is part of what can help an autocrat/extremist/demogogue gain power: Citizens United, billionaires, rich sponsors, campaign finance laws and grassroots fundraising esp. in the U.S..

But even with money, there is still a way to prevent him from gaining power. Once he has it, preventing an autocracy.

"By choosing Bernie over Biden, Buttigieg, Klobuchar and most certainly Bloomberg and Steyer.
Answer not limited to the U.S..

Hint 1: "By choosing" has to do with preventing him from gaining power. By choosing what and by whom?

Hint 2: It was done successfully in the past in the U.S.. More recently in foreign countries.

Nope.

Nope.

Nope.

"-sometimes its a small dedicated armed ban who seizes power"

True. During the Cold War, 3/4 of democracies ended in a coup or coercion. Furthermore, most governments became an obvious dictatorship like communism, fascism or military rule. Coups are rare now.

"-sometimes a majority of the population support them
-sometimes they take power under the guise of a democratic party
-sometimes they admit they are going to be a strongman
-sometimes the seizing of power is over a short period of time
-sometimes its a slow steady securing of power through a thousand cuts."

All true. You listed the ways a democracy normally ends now.

"how there's one answer."

What's been the common denominator or main way an autocrat/demagogue/extremist, etc. is prevented from gaining power or subverting democracy once in power?

Take a guess. You may be right.

You're heading in the right direction!

WHO wouldn't allow the head of state to make his/her own rules?

You may solve this.