There need not be much integrity for a monarchical or despotic government to maintain or sustain itself. The force of the laws in the one, and the prince’s ever-raised arm in the other, can rule or contain the whole. But in a popular state there must be an additional spring, which is virtue.

[Il ne faut pas beaucoup de probité, pour qu’un gouvernement monarchique, ou un gouvernement despotique, se maintiennent ou se soutiennent. La force des loix dans l’un, le bras du prince toujours levé dans l’autre, reglent ou contiennent tout. Mais, dans un état populaire, il faut un ressort de plus, qui est la VERTU.]

Charles-Lewis de Secondat, Baron de Montesquieu (1689-1755) French political philosopher

Spirit of Laws [The Spirit of the Laws; De l’esprit des lois], Book 3, ch. 3 (3.3) (1748) [tr. Cohler/Miller/Stone (1989)]
    (Source)

In his Preface, Montesquieu clarifies:

For the understanding of the first four books of this work, it must be noted that what I call virtue in the republic is love of the homeland, in other words love of equality. It is not a moral virtue, nor a Christian virtue, it is political virtue; and this virtue is what drives republican government, as honor is what drives monarchy. I have therefore called political virtue love of the homeland and of equality.

(Source (French)). Other translations:

There is no great share of probity necessary to support a monarchical or despotic government: the force of laws, in one, and the prince’s arm, in the other, are sufficient to direct and maintain the whole. But, in a popular state, one spring more is necessary, namely, virtue.
[tr. Nugent (1750)]

It does not take much probity for a monarchical or despotic government to maintain or sustain itself. The force of the laws in the first, and the ever-threatening arm of the prince in the second, determine or contain everything. But a popular state needs to be driven by something more, which is VIRTUE.
[tr. Stewart (2018)]