Over the past six weeks on his social media platform, Truth Social, Mr. Trump has been posting videos about his policy positions, including plans to protect Social Security and Medicare and ban Chinese nationals from owning U.S. farmland or telecommunications, energy, technology or medical supply companies. The videos, in which the former president speaks directly to the camera, are aimed at reassuring supporters that he’s focused on topics other than his 2020 defeat, an issue that flopped with midterm voters. One of the key trouble spots for him has been in attracting top-dollar backing. Mr. Trump has largely relied on...