44 Furthermore, it is in general fitting and salutary that Catholics should extend their efforts beyond this restricted sphere, and give their attention to national politics. We say "in general" because these Our precepts are addressed to all nations. However, it may in some places be true that, for most urgent and just reasons, it is by no means expedient for Catholics to engage in public affairs or to take an active part in politics. Nevertheless, as We have laid down, to take no share in public matters would be as wrong as to have no concern for, or to bestow no labour upon, the common good, and the more so because Catholics are admonished, by the very doctrines which they profess, to be upright and faithful in the discharge of duty, while, if they hold aloof, men whose principles offer but small guarantee for the welfare of the State will the more readily seize the reins of government. This would tend also to the injury of the Christian religion, forasmuch as those would come into power who are badly disposed toward the Church, and those who are willing to befriend her would be deprived of all influence.
Source: Immortale Dei (Vatican.va)