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'.
(..) - Well, I'm still dancing. I'm glad you're finally here. Charlie here hasn't shut up about it since you told him you were coming. All right, keep exaggerating. I'll roll you into the mud. After I ram you in the ankles. You want to go? Bring it. - Hi, I'm Jacob. - Hey. We used to make mud pies when we were little. Right. No, I remember. Are they always like this? - It's getting worse with old age. - Good. (..)2008 Twilight