The senatus consultum ultimum is a modern term applied to, as you say, a state of emergency declared by senatorial review. Generally it was a consultum, the usual advisory report issued to the magistrates, directed at the consul or a suitably equivalent magistrate and intended to remind the consul of his duty to prevent seditio. Legally speaking there were effectively no restraints to the consul's powers, barring the refusal of cooperation by his fellow magistrates and subordinates. The SCU therefore acted similarly to a domestic dictatorship, and was introduced after the lapse of the dictatorship--one early twentieth century German scholar called the SCU "die Quasidiktatur" (Plaumann, in Das sogenannte S.c.u., die Quasidiktatur der späteren römischen Republik).

The deal with the Gracchans is that, in sum, 121 was the first time the SCU had ever been used. It was a novel legal principle, and Opimius' acquittal essentially served to establish it as a valid senatorial recourse against seditio, with the Gracchans serving as the precedent--Opimius was acquitted because there was no precedent to accuse him of breaching. Of course, Opimius might have been condemned, but enough of the nobility supported the concept of an extralegal means of combating tribunician sacrosanctity that the precedent was established. The SCU underwent a fair amount of development following 121, however. Opimius' invocation of the consul's duty to prevent seditio consisted, essentially, of the temporary revocation of the individual's right of protection against coercitio. The earliest uses of the SCU appear not to have included a formal declaration of the target as hostis, and it was not until Sulla outlawed Marius that the SCU formula included a formal declaration of the target as hostis (usually when the individual was physically outside the city, as Catiline was in 63 and Caesar would be in 49). Attempts were made to more rigorously define the conditions necessary to declare a state of emergency by SCU, most notably the lex Lutatia de vi and the lex Plautia de vi, according to which the Catilinarians were accused. But these were generally unsuccessful, and both the conditions necessary to declare potential seditio and the limits of the consul's powers remained fairly poorly defined throughout the first and second centuries.

You bring up Cicero's condemnation to exile, and that's an interesting case, and something of an anomaly. Trials in the wake of the enactment of SCU were not unknown, starting with Opimius but including, for example, Rabirius (many years later)--Sulla might have been brought to trial had he not seized the courts. The charge was, naturally, violation of the citizen's right to trial before execution, which derived at least in part from his protection from coercitio--whether the citizen really had this right after declaration of SCU is something that has puzzled scholars for quite some time, and the answer seems to be that it kinda depended on what the court thought at the time. In the case of Opimius they obviously thought the consul was in the right. In Rabirius' case the comitia were at best deadlocked and at worst leaning towards condemnation for perduellio, until Metellus Celer dissolved the comitia by lowering the Janiculine flag, ending the vote prematurely. Cicero, of course, was condemned. But Cicero was a somewhat special case. Cicero's imprisonment and execution of the Catilinarians featured a number of oddities. The first was that Catiline was not actually in the city, he was mustering an army in Etruria--only Lepidus had been outlawed by SCU from outside the city before. Moreover, the Catilinarians within the city, where SCU usually was invoked, had been imprisoned. The Gracchans had refused their right to trial and had taken up arms instead. Saturninus and Glaucia were murdered without trial, but the circumstances of that event are unclear. Both Appian and Plutarch, our two major sources, note that Saturninus and Glaucia had surrendered to Marius, but Appian says that the mob murdered them while they were awaiting trial, whereas Plutarch simply states that Marius had them executed--it is not mentioned whether they had a trial or not, although Plutarch notes that Marius tried to prevent the death penalty. In the case of the Catilinarians Cicero was looking at a group of guys already imprisoned, who themselves represented apparently little danger. Cicero's case was that the Catilinarians presented the danger of riots in the city intended to break them out, but apart from a small group of Lentulus' freedmen and some hired muscle no public disturbances ever materialized. This lack of actual tangible threat weighed heavily on the minds of both the plebs and the nobiles, as did a provision that Cicero attempted to pass, with debatable success. The Catilinarians had been arrested without having actually carried out much in the way of real violence. Cicero had plenty of evidence to bring them to trial if he had wanted to--and the suggestion was nearly successful, until Cicero and Cato brought up the idea that letting the Catilinarians live was inviting violence--but he treated them as hostes (as Catiline himself had been declared in November) on the basis that protection against coercitio was suspended not only on the grounds of the power granted the consul by SCU but because citizenship should be automatically lost in the case of treasonous activity.

Something else to note is that Cicero's eventual exile took place under very unusual circumstances. The balance of opinion among the nobiles was hardly unified, and there was strong dissent in the senate, to say nothing of the plebs. Already by the end of the year Cicero was facing the result of ignoring these dissenting opinions when he was prevented by tribunate veto from delivering his farewell address as consul. His actual exile was only imposed years later, and it was brought up as much to attack him personally as as a legitimate point of legal dispute. By 58 Cicero had maneuvered himself into a very awkward position. He had presented himself as an opponent of quasi-dictatorial and extraordinary rule, but had strong-armed the senate into executing the Catilinarians. He had supported Pompey strongly before the Mithridatic War, but found himself in a somewhat embarrassing position when Pompey returned from the east and began terrorizing the senate. He had made a powerful enemy for himself in the person of Publius Clodius, although I don't think he can be faulted for not realizing how much damage Clodius would be able to do down the line. In 60 he had rejected Caesar's offer of entry into the triumvirate, and again refused to accompany Caesar into Gaul in 59. Moreover, Cicero put his foot in his mouth by beginning to speak out against Caesar's consulship in 59, after agreeing to keep silent--Caesar's sudden support for Clodius was a result of this indiscretion. And Clodius and Pompey in 58 managed to turn over the most powerful of Cicero's old friends, most importantly Cato, who failed to oppose the motion for exile as Cicero expected him to--in the same way that Pompey personally brought Cicero back from exile months later, Pompey's influence was crucial in the motion to outlaw him in the first place, a stab in the back that Cicero tried to rationalize through tortured logic for years. One other important point, perhaps the most important, is that Cicero's exile was of questionable legality. Cicero was not exiled by court or even brought to court. His actions were first declared illegal by the lex Clodia de capite civis Romani which reaffirmed the Roman citizen's right to provocatio. The law did not technically name Cicero, but it was clearly aimed at Cicero's violation of provocatio in the case of the Catilinarians--in particular the law allowed the citizen body as a whole to invoke provocatio retroactively and without trial, by voting to deny fire and water by vote, and without allowing for prosecution and defense, to anyone specifically who had executed Roman citizens without trial. This provision, Cicero later argued, was illegal, although he probably wasn't right in particular that it was a privilegium, a bill illegally naming and directed against a particular individual and no one else. However, following Cicero's voluntary exile Clodius passed the lex Clodia de exsilio Ciceronis. This was undeniably an illegal privilegium. The law named Cicero and prescribed, retroactively, the penalty of exile, yadayadayada. Now, technically speaking this was normal--exiles in Roman law were usually voluntary flights from court, and were affirmed by special legal proceedings afterwards. But Cicero had never been brought to court or even accused. The legal grounds by which Clodius was able to pass a tribunate motion for exile were shaky indeed, as usually the special act of confirming legal exile was carried out as an extension of court proceedings, after it was found that the defendant was no longer showing up--this was demonstrably in violation of the Twelve Tables. Cicero also claimed that the law was in violation of the lex Caecilia et Didia and the lex Licinia, but we don't know enough about those laws to know why, and he doesn't explain what parts of the laws he claims Clodius violated.

CONT., I don't think I've written a post this long in like a year...