Caesar's major obstacle were the optimates. Caesar was a
popularis, which was a political faction which championed the cause
of the poor and wanted to introduce reforms in their favour. The
populares (plural of popularis) were opposed by the optimates, a
conservative political faction which favoured the aristocracy and
opposed the reforms. This aristocracy had also came to be seen as
corrupt.
Caesar gained power by fighting a civil war against the
optimates. Many of assassins were optimates.