Obviously the media can't change the mind of people who are already heavily engaged in politics, like on this forum, but it can persuade many casual voters who pay much less attention to politics to vote for their man.
Not in the way you think it does. First of all, people who are only barely interested in politics will only barely follow media coverage of political events, be it pro-Obama or anti-Obama. At best, reports from the presidential campaign trail will cause them the switch channels. And like I just explained to Jacobtm, voters are primarily affected by their daily lifes and the "millieu" they tend to life in.
"Propaganda" only works if you
want to believe it. So, pro-Obama propaganda only works with Obama supporters. Its purpose is the solidify the own base.