Originally Posted by walbertsmith
On top of what JGS mentioned, think about the words "rightful throne." He sent out that letter to everyone in the country, yet he's struggling to field an army (resorting to unsavory types like pirates and Melisandre)... and even if every single person in the kingdom believed that Joffrey was not the rightful king, then undoubtedly Stannis should be king, right? Yet here stands Renly with an incredible force of a 100,000 strong behind him.
This theme is repeated again and again this season... power is not in words, or religion, or military might... power resides in those who the people perceive have power.
But the theme is not that simple, the theme is that different people have different ideas on what power is and it still remains to be seen who's right.
So far this season we have seen:
Littlefinger: Knowledge is Power.
Cersei: Power is Power.
Varys: Power is an illusion.
Dany: Power is in her Dragons.
Renly: Power is in having friends.
Mellisandre: Power is in the supernatural.
Who's right, and who's wrong? I think Cersei is clearly wrong. But all the other answers are probably correct to an extent.