This question recently appeared in one of the AQA classical civilisation exams and I answered a similar question to this in my 'Roman Republican History: 146 - 46 BC' final exam at Oxford University, for which I was awarded a First.
When planning a response to this question it is probably best to deal with the changes to the tribunate in a chronological manner. These changes are closely linked to narrative of the Late Republic and this context may be lost if a thematic approach is adopted.
Before 287 BC the senate had enjoyed a legal 'check' over the tribunate, it essentially meant only legislation that the senate had approved could be proposed by a tribune to the plebeian assembly. This legal check was removed with the lex Hortensia of 287 BC, however, the senate had managed to keep interpersonal checks on the tribunes meaning direct challenges from the tribunes largely did not occur immediately after this legislation.
Gracchi and other 2nd C BC tribunes:
This, of course, changed with Tiberius Gracchus’ tribunate in 133 BC. Tiberius openly defied the senate with his own popularity. He vetoed any action of government until his agrarian bill was addressed, had the tribune Octavius unconstitutionally removed by a vote in the assembly of the plebs, had his bill passed and then returned to the assembly in order to use funds left by the King of Pergamum for his commission. This was a challenge to the senate as they enjoyed sole influence over foreign matters. As his term of office was nearing end, thus leaving him open to prosecution, he simply stood for a second, unconstitutional, term. Tiberius here was not only violating but gravely upsetting the balance of the constitution, his will essentially became law due to his popularity, which bypassed the senate and the elected executive magistrates.
It was not just the Gracchi that were using the power of the plebs to defy the senate in this period: In 131 BC, C. Papirius Carbo proposed that elections should be held by secret ballot and in 104 BC L. Cassius Longinus passed several laws to limit the power of the nobility, including one that reaffirmed the people’s right to abrogate the power of a magistrate.
Tribunes using their power to support already powerful men – end of 2nd C / start of 1st
In these final years of the second century BC, while we see the tribunes challenging the power of the senate, they are at least doing this in order to advance the interests of the plebians, or at least a political agenda that benefits the plebians. However, at the turn of this century we see tribunes begin to use the power of the plebs to advance the private political agendas of already over powerful individuals. Saturninus (103-100 BC), for example, arguably entered into a quasi-triumvirate with Marius and Glaucia, in order to complement Marius’ military power with his own popular power. Furthermore, Marius had a tribune revoke Sulla’s military command of the war against Mithridates.
Sulla castrates the tribunate – a response to political exploitation of popular power?
Through his constitutional reforms Sulla forbade the tribunate from proposing new legislation or being able to veto the acts of the senate. Moreover, he reduced the prestige of the office by banning tribunes from holding further political office. Sulla, then, clearly made sweeping changes to the role of the tribunate
Pompey and Crassus restore the tribunate, 70 BC, tribunes now firmly part of late republican popular politics and political machinations.
With the restoration of the tribunate under the consulship of Pompey and Crassus, the office became fully exploited by the ‘big men’ of the late republic for their own political gain. Pompey was almost immediately voted his extraordinary command against the pirates by the very tribunes he had restored, Caesar had Clodius’ status changed from patrician to plebeian so that he could hold tribunician power and in the lead up to the civil war the partisan legislation and counter legislation by tribunes supporting Caesar and Pompey can clearly be seen.