How did Gaius Marius become tribune of the plebs in 119 BCE?

Gaius Marius became tribune of the plebs in 119 BCE with the political backing of the Caecilii Metelli, a distinguished and powerful noble family that enjoyed patronage over his own family.1

During his time as tribune, Marius demonstrated his political ambition by proposing a bill intended to reform election procedures. The legislation aimed to narrow the "bridges"—the gangways voters used to cast their ballots—to prevent aristocrats from acting as observers and influencing individual votes. When the consuls attempted to have the Senate block this measure, Marius successfully forced the bill through by threatening the consuls with imprisonment.1