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'.
(..) - Yeah. Yeah, that's it. Let's get out of here. Here, use this. - What are you doing? Aah! - Let's go! Quit playing around in the mud. I could use your help. Wyldstyle, we could really use... Vitruvius, they're gaining on us. Build something! (..)2014 The Lego Movie