Populism is a range of political stances that emphasize the idea of the common people and often position this group in opposition to a perceived elite group.

https://en.wikipedia.org/wiki/Populism

political ideas and activities that are intended to get the support of ordinary people by giving them what they want:

https://dictionary.cambridge.org/dictionary/english/populism

a political philosophy or movement that represents or is claimed to represent the interests of ordinary people especially against the Establishment

https://www.merriam-webster.com/dictionary/populism

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You’re probably thinking of a demagogue:

a political leader in a democracy who gains popularity by arousing the common people against elites, especially through oratory that whips up the passions of crowds, appealing to emotion by scapegoating out-groups, exaggerating dangers to stoke fears, lying for emotional effect, or other rhetoric that tends to drown out reasoned deliberation and encourage fanatical popularity.

https://en.wikipedia.org/wiki/Demagogue