What prevents a democracy from becoming an autocracy?

WE DO.

dem

ANSWER: Mainstream political parties

Explanation:
1) Political parties keep extremists/autocrats/demagogues off the mainstream party ticket. They refuse to endorse or align with them, and if needed, align with political opponents. Money, alliances and endorsements are denied. Fear, opportunism and miscalculation like the belief that an extremist, etc. can be controlled can endanger democracy. Bringing them into the mainstream gives them power and legitimizes them.

Examples: Hitler and Venezuela's Chavez were brought into political mainstream by mainstream political parties with belief they could be controlled.

Political parties stopped George Wallace, Henry Ford, France's Marine LePen and Austria's Norbert Hofer.

If the aforementioned fails, then

2) Political parties must exercise certain behaviors: forbearance (restraint when a police party has temporary majority by accepting other parties legitimacy and maintaining political and democratic norms. In other words, not taking advantage. Maintain the checks and balances and follow the political rules and norms that have long been established by the parties.

Extreme partisanship helps destroy democracies: obstructionism, refusing to follow long established government norms & rules, weaker checks and balances, voter purges, neutral institutions like courts are weakened, refusal to see political opponents as legitimate therefore refusal to work with them, collective political party abdication aka: transfer of power to a leader who threatens democracy.