Trump posted a threat on his social media site saying: "The easiest way to save money in our budget, billions and billions of dollars, is to end government support for Elon and his contracts".

And if Trump directs government agencies against Musk, the tech billionaire will feel the pain. Tesla's stock price fell by 14 percent on Thursday.

But Musk also has nearly limitless resources to fight back, including funding rebellious Republican challengers in next year's elections and primaries. He may not win a battle against the entire Trump government, but he could impose a heavy political cost on him.

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