INTERCESSIO. — In Roman public law, *intercessio* begins with the founding of the Republic and initially signifies the right of one of the two consuls to prevent, by virtue of their *par potestas*, a legal act performed by the other consul from taking effect.
The concept of *intercessio* is closely tied to that of collegiality. When new magistracies were later created, the right of *intercessio* was exercised not only between colleagues but also by a superior magistrate, by virtue of his *maior potestas*, against measures taken by an inferior magistrate. Thus, a consul could intercede against the acts of any other magistrate, and a praetor could do the same against magistrates subordinate to him. However, while higher magistrates could intercede against those of equal rank in the *cursus honorum* and against lower magistrates, among the latter—though hierarchically ordered—no *intercessio* took place. The censor occupied a special position, as he could neither intercede against others nor be subjected to *intercessio*.
To these limitations of power among magistrates, the establishment of the plebeian tribunate in 487 BC added another form of *intercessio*, which became the most frequently used. The plebeian tribunes were not magistrates but, endowed with *ius auxilii*, could intercede in defense of plebeian interests against the acts of any magistrate (except, of course, the dictator), against decrees of the Senate (*senatus consulta*), and against legislative proposals (*rogationes*) presented to the assemblies for a vote. This *intercessio* of the tribunes saw the widest application. It is understood, however, that such a right could only be exercised within the city and its immediate vicinity. Acts performed by a magistrate possessing *imperium* to be exercised outside the *pomerium* were therefore exempt from *intercessio*.
In the Empire, the right of *intercessio* belonged first and foremost to the Emperor by virtue of his *tribunicia potestas*, and in a broader form than before—not only because the *tribunicia potestas* became a lifelong power but also because it extended not merely to the city of Rome but to the entire Empire. Moreover, it could not be countered by the *intercessio* of another plebeian tribune, since there was no collegiality between the plebeian tribune and the princeps invested with *tribunicia potestas*. In the 1st century of the Empire, instances of imperial *intercessio* are recorded, and much more rarely, of plebeian tribunes against the acts of magistrates. Eventually, the latter form of *intercessio* was suppressed, while the former became superfluous with the consolidation of imperial absolutism.