The right of intercessio applied, theoretically speaking, to a consul's colleagues and to any magistrate considered junior to him, although the intercessio of tribunes applied to senior magistrates as well. However, in all practice the tribunate alone maintained a working veto. Though theoretically the consuls had the power of intercessio over lower magistrates (known, for example, from CIL 2.1963) in fact there are no clear instances when a consul vetoed a praetor or lower. And indeed instances when a consul vetoed his colleague are exceedingly rare--I cannot think of a single one off the top of my head.

Strictly speaking neither the tribunes nor the consuls could veto the legislation of the senate or the assemblies, in the first place because neither of them had the power to propose legislation. The senate was not a legislative body, but an advisory one, and while its consulta were extremely influential and often served as the basis for laws later taken to the assemblies they were not in fact law. Only the assemblies had the power to enact legislation, but they lacked the power to promulgate it. Bills were instead promulgated by magistrates for ratification (or, sometimes, rejection) by the comitia. As such the assemblies had no independent decision-making power to which a veto might be applied. The comitia could be dismissed or the magistrate's right to promulgate his law vetoed, but the actual comitia was not a magistrate and could not actually be vetoed. It could, of course, be rendered useless by vetoing the presiding magistrate's ability to promulgate his bill, in which case the comitia just sat around doing nothing. In 133 Octavius repeatedly vetoed Ti. Gracchus' right even to read his agrarian bill out loud, in response to which Gracchus carried through the comitia the abrogation of Octavius' magistracy. If we are to believe Plutarch he persuasively argued that the tribune's job was to protect the rights of the people, first and foremost among them their right to vote on legislation--by preventing the people from exercising their voting rights in the first place Plutarch's Ti. argues that Octavius had, for all intents and purposes, already resigned from his magistracy himself. Octavius' case was unusual, however, and thus provoked Ti. Gracchus' particular justification for his actions. More usually in the face of a legislative veto tribunes either withdrew their bills or, in the event that popular opinion was manifestly against the vetoing tribune, the veto was withdrawn. There was ideological baggage attached to the office of tribune. By contrast, we do not find consuls vetoing the promulgation of bills by their colleagues or lower magistrates. I do not know of a single occasion on which such a thing happened. Consuls could and sometimes did use their enormous auctoritas to put a lid on things, or used their exclusive right to the auspices to break up the comitia. Famously in 200 the Roman people, tired out by the Punic War, rejected a motion to begin a new war in Macedonia, until the consul Galba halted the comitia and asked them to change their mind. The consuls also could declare various types of public holidays to prevent voting, as Sulla and Pompeius Rufus did against Sulpicius, who declared the action unlawful and forced the consuls to rescind the recess by violence. But such actions were unusual and not normally tolerated--no matter what his technical rights the consul quite clearly did not have the ideological high ground when he prevented the people from using their vote.

It's tempting to think of intercessio as being some silver bullet that was used constantly to prevent opposition from ever raising its voice. But apart from the limitations on intercessio it seems quite clear that in fact the ideological framework of the Republic--a hotly debated topic--prevented its frivolous use. It is notable that even during the tumult surrounding Caesar's legislation in 59 the idea of Bibulus exercising a veto is never once raised in our sources, despite his clumsy attempt to prevent voting by obnuntatio.