In common with virtually all thinkers of the time, Thomas Aquinas was keen to ensure the general stability of society. Whether it was sudden eruptions of violent disorder from the populace or the unjust, brutal reign of tyrants, they were equally anathema to this peaceable soul.
Despite his avowed abhorrence of social and civic disorder, however, Aquinas's political philosophy, according to critics, does not provide an adequate means to challenge the main source of such disorder, namely the rule of tyrants. Although Aquinas strongly positions himself against tyrants, his recommendations for dealing with them have struck successive generations of scholars and critics as being woefully unrealistic and ineffectual.
The main reason for his lies in Aquinas's extreme squeamishness concerning armed rebellion against a tyrant. Not unreasonably, perhaps, Aquinas is worried that any armed uprising against a tyrant, no matter how justified it may be in theory, is bound to destabilize the realm, leading to chaos and disorder. That being the case, he argues that the appropriate response of the people towards tyranny is to withhold obedience from the relevant tyrant.
To be sure, Aquinas allows that the forced removal of a tyrant is appropriate in certain circumstances, but those circumstances would have to be pretty extreme. Even then, the toppling of a tyrant could only be carried out by subordinate magistrates, those who work within the political system, and not the people themselves.
As for the ordinary people, the most they can do, as we've already seen, is to withhold their obedience from the tyrant. In practical terms, however, it's difficult to see how this would work, as it would undoubtedly lead to large swathes of the population being terrorized and cowed into submission by a tyrant angered by their insubordination.