As most users haven’t actually asked the titular question, I’ll take a stab.
First, keep in mind that there are multiple, competing theories on why authoritarian leaders rise from democratic systems. An entire branch of political science / political philosophy is often devoted to asking this question. These theories (broadly speaking) can include theories such as public choice, rational choice, Marxist, critical race, post-structural / post-modern, feminist, postcolonial, emotional choice, and the field of political psychology (which in itself may disagree on explanations but nonetheless seek them). These theories tend to overlap with economics and sociology, so explanations may be investigated in different ways within different social sciences. Additionally, there is also much debate on the effect of leadership styles in authoritarian movements. Some argue that individual leaders are just a symptom of a greater problem in society, while others argue that individual leaders hold the blame for directing these problems towards an authoritarian goal. Lastly, theorizing an explanation to this question is also impacted by one’s own ideology.
I would argue that there is no one, perfect, overarching explanation for why every authoritarian figure rises to power in a multi-party parliamentary system. The history and context of individual political systems, societies, and nations can have different effects and consequences. It’s probably a combination of an individual authoritarian leader’s traits and the society that gave birth to them. That being said, I tend to favour “critical” theories rather than “descriptive” theories, as these theories try harder to get at the real root of the problem your question addresses: where is the power in a society, and why is power organized (or reconfigured) in this manner? And, from that, I tend to see authoritarian leaders as more often a result of the power dynamics within a society rather than just “one bad apple”.
Notes on Definition
There are also some complications with answering more specifically to the system you have described. First, a “multi-party system” can still include first-past-the-post, as it is the norm in several countries with first-past-the-post to have more than two political parties elected. There is also some gray area on whether certain figures would be included under this definition. Adolf Hitler, for example, came initially to power and further gained dictator status through a semi-presidential system, whereby the president is elected separately as the nation’s executive. He, however, effectively engulfed the presidential role when he rose to power, thus ending the system. In my view, this would exclude him being included as an example for answering your question, as the executive was technically accountable first to the electorate directly and not the legislature during his formative years that allowed for him to rise to power in the first place.
Instead of providing a list of examples, here is a list of catalysts for growing authoritarianism in any political system. These aren’t “necessary” conditions for an authoritarian leader’s success, but the more of these you check off in a given environment, the more likely an authoritarian leader will be successful:
- Poor economic conditions for a large proportion of the populace
- Growing nepotism, bribery, extortion, intimidation, and other forms of corruption
- The murder, imprisonment, exiling, or ostracization of professions encouraging the free exchange of ideas (i.e. teachers, journalists, activists)
- Large or growing political apathy
- Public acts of “othering” to distinguish an in-group from an out-group, such as harassment, hate speech, threats, defacement of property
- The consolidation of mass media and other forms of expression to the benefit of the authoritarian group-in-question
- Increasing use of violence to resolve political disputes
- Growing use of formal or informal practices (such as religion, ideology, or law) to eliminate political opposition or curtail dissident behaviour
At the end of the day, a political system is only as good as the society that cultivates it. Political systems are imagined – and contested - realities. If a society cultivates attitudes and traits that encourage violence, factionalism, bigotry, and desperation, then it should not be surprised when its leaders turn out to embrace these things.