(CNN) A top adviser to Donald Trump resigned Monday after sending a mocking tweet about just-fired campaign manager Corey Lewandowski, a campaign source tells CNN.

Michael Caputo, a Trump adviser and head of the communications for Trump's caucus operations team, tweeted "Ding Dong the witch is dead" following the news that Lewandowski had been fired.

"I regret sending out a tweet today alluding to the firing of Corey Lewandowski. In hindsight, that was too exuberant a reaction to this personnel move. I know this is a distraction from the kind of campaign you want to run, so I'm resigning my position as director of communications for caucus operations at the 2016 Republican National Convention. Let's make this immediate," Caputo wrote in a letter to campaign chairman Paul Manafort and Rick Gates, according to a copy obtained by CNN.

Caputo also said he deeply respects Trump as well as Manafort and Gates.

"I wish you success in the months ahead. I continue to believe Donald Trump must be elected President in November. With you at the helm, I know he can. Call if I can help," he said.

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