Historically, why does Brutus have a problem with Julius Caeser's power?

Brutus was primarily concerned with the consolidation of political power by Caesar and the threat that it presented to the Roman Republic. Brutus saw his allegiance as being to the Roman Senate. By declaring himself the ultimate political authority of Rome and continuously usurping further power from other Roman institutions, Caesar had undermined the ability of the Senate to effectively govern. Brutus came to see Caesar as a tyrant, an oppressive ruler who was distinctly separate from and no longer subject to the citizenry of Rome. The assassination of Caesar was viewed as necessary to secure the Republican nature of Rome, and as it was to this principle which Brutus had his fundamental allegiance, he considered it to be his duty to oppose Caesar by whatever means he was able.