It probably won't cost him today but Trump's mishandling of the David Duke endorsement is worse than just the prima facie stupidity of blaming the earpiece.
It's strange that Trump, whose great strength in the polls derives from his being perceived as the bane of a corrupt and moribund GOP establishment, who has scored points during debates by calling out "lobbyists" booing him in the audience, would not have thought to simply ask how much the RNC or Karl Rove or whoever paid Duke for the endorsement. That way Trump wouldn't have to "embrace or renounce" Duke and could instead use the situation as a means to swipe at the faceless minions of.. whoever.. who are out to get him.
Besides, it's probably true. David Duke is a hate propagandist first. But professionally speaking he is also a con man among con men dabbling in tax evasion and mail fraud and "fundraising" off of his own infamous brand. There is practically zero reason to believe Duke's endorsement of Trump wasn't paid for by somebody. It's remarkable that Trump wouldn't immediately recognize Duke's grifting style for what it is. I mean, it takes one to know one, right?