Under US federal law, it is illegal to give payments that are intended to influence the way a person is voting.
However, some experts have said the offer made by Trump in Texas on Wednesday could be seen as a pledge to cut taxes, which is legal.
Dr Joan Mahoney, lecturer in law at the University of Southampton, told the BBC she thought the offer would be legal, though she said politicians “don’t usually do it quite this explicitly”.
“On the one hand it sounds a bit like a bribe, vote for the Republicans and you’ll get money, which would be decidedly illegal,” she said.
“But on the other hand it would go to everyone, regardless of how they voted so it isn’t in that sense a bribe.
“I think it would be legal. It’s the same kind of campaign pledge as saying ‘if my party wins we’ll reduce taxes’. Every party does that all the time… and that’s obviously perfectly legal.”
However, Laurel Rapp, director of the US and North America programme at Chatham House, told the BBC that Trump’s offer seemed to be “a very different, very transactional agreement” from the kind of promise usually made by candidates in an election period.
