To become the Supreme ruler of Rome, you will need a huge dominion which of course is bigger than you rivals. Also, watch your standing with the locals. See the image that you posted?Click on the Senate tab and click on the Senate floor tab. there you will see your standing among the Senate and the Locals. All you have to do is win many a battle to raise your rep among the locals in Rome. One thing I noticed is that it's almost automatic that when my rep among the mob becomes high, the Senate becomes displeased(probably envious of my achievements) so no need to worry. As an ultimatum(in my case), the Senate gave me a mission in which my faction leader was to commit suicide. I refused so I got outlawed and that was my go signal to land my invasion army that I was formed up in Greece.(I was the Brutii).