The candidates started 'slinging mud' (at each other) early in the campaign.
His shoes were covered with 'mud'. = His shoes were caked in/with 'mud'.
The scandal ruined his reputation and now 'his name is mud'.
(..) -That's what I hear. I'm gonna tell you, Gene, if you're gonna make a play, look east. I'd rather it be you than someone else. This mud around here has been taken. It must be easy when you have such a cute face to carry around with you. -Telling me how handsome my son is? -Well, that I am. (..)2007 There Will Be Blood
(..) We're making the real money. What we should be making. Not just... Not just throwing it away on shipping costs. Otherwise, it's just mud. How much are we going to pay them? Who's that? The Sunday family. I'm not going to give them oil prices. I'll give them quail prices. (..)2007 There Will Be Blood