Mencius (Mengzi), answered from the texts and cited to the page.
The mandate of Heaven (tian ming) is bestowed on the ruler whose conduct serves the people; it is withdrawn from the tyrant whose conduct harms them. The people's welfare is the test of legitimate rule, not the ruler's power or his lineage or the elaborateness of his court. The famous 7B:14 doctrine states the priority plainly: the people are most important; the altars of grain and soil come next; the ruler is least important.
The radical implication is in 1B:8. King Xuan of Qi asked me whether the Zhou founder's killing of the Shang tyrant Zhou was regicide. I replied that one who mutilates benevolence is called a mutilator, one who mutilates righteousness is called a crippler, and a mutilator-and-crippler is called a mere fellow. I have heard of the killing of the mere fellow Zhou, but I have not heard of any regicide.
The tyrant who has lost the mandate is no longer a king; the killing of him is not regicide. The kingly way (wang dao) of benevolent government, ritual propriety, and concern for the people's welfare is the legitimate path. The hegemonic way (ba dao) of rule-by-force, military conquest, and fear is illegitimate, regardless of how successful it may be militarily.
Heaven sees as the people see; Heaven hears as the people hear (5A:5).1 The judgment of legitimate rule is finally the judgment of the people, through whose welfare the mandate of Heaven becomes manifest.23
This sentiment is expressed in the words of The Great Declaration: "Heaven sees according as my people see; Heaven hears according as my people hear."' Wan Zhang asked Mencius, saying, 'People say, "When the disposal of the kingdom came to Yu, his virtue was inferior to that of Yao and Shun, and he transmitted it not to the worthiest but to his son." Was it so?' Mencius replied, 'No; it was not so.The Works of Mencius
Both these cases are the rule of Heaven. They who accord with Heaven are preserved, and they who rebel against Heaven perish. The duke Jing of Qi said, "Not to be able to command others, and at the same time to refuse to receive their commands, is to cut one's self off from all intercourse with others." His tears flowed forth while he gave his daughter to be married to the prince of Wu.The Works of Mencius
Among the ancients there was one who acted on this principle, namely king Wen. When, with all the strength of your country of ten thousand chariots, you attacked another country of ten thousand chariots, and the people brought baskets of rice and vessels of congee, to meet your Majesty's host, was there any other reason for this but that they hoped to escape out of fire and water?The Works of Mencius
The mandate-of-Heaven doctrine is one of the most influential political-philosophical positions in East Asian intellectual history. The framework is older than Mencius — the Book of Documents records the early Zhou mandate doctrine; Confucius referenced it; the entire Western Zhou political theology presupposed it. But Mencius's contribution was to make the people's welfare the explicit test of the mandate, to derive the regicide-is-not-regicide conclusion from the framework, and to articulate the kingly-vs-hegemonic distinction that became foundational to Confucian political theory. The Han Confucian synthesis (Dong Zhongshu) developed the framework into a state-theological system. The Song Neo-Confucian revival made Mencius's political theory central to East Asian governance for nearly a millennium. Contemporary engagement: the New Confucian Chinese- language tradition (Mou Tsung-san, Tu Wei-ming) carries forward the Mencian political legacy; the East Asian democratic-legitimacy debates (Daniel A. Bell, Joseph Chan, Sungmoon Kim) connect Mencius's people's-welfare framework to contemporary democratic theory.