The world’s richest person has quietly become by far the biggest known midterm donor, trying to save Republicans’ Senate majority with some of the largest donations in American political history.

Elon Musk has put at least $276 million into Republicans' midterm hopes since July.

Musk's largesse, which was reported earlier by the New York Times, makes the world's richest person likely the single-biggest political donor in this year's races, according to campaign finance record...