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Donald Trump 's sharing a stern warning with Elon Musk ... implying he's better off staying out of politics entirely than supporting Democrats in the upcoming midterms.

The President of the United States drew the line in the sand for his former advisor during a phone interview with NBC News ... telling them he would bring the smackdown on EM if he supports Dems instead of Republicans that vote for Trump's tax and spending bill. Getty

DJT said, "He’ll have to pay very serious consequences if he does that."

It's just the latest threat in a back-and-forth between Trump and Musk ... coming a few days after Elon blasted the prez for the spending bill he felt was filled with pork -- claiming President Trump only won the reelection because of him . Gallery Launch G

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