Trump, who opposes witnesses, has both carrots and sticks to offer wavering Republicans. One Trump confidante told CBS News that GOP senators have been warned: 'Vote against the president, and your head will be on a pike.'

Trump has also been 'rewarding senators who have his back on impeachment' by helping them raise campaign cash, 'and sending a message to those who don't to get on board,' Politico reported in October. And Trump, U.S. Chamber of Commerce strategist Scott Reed noted, 'has the ability to turn on the money spigot like no one else.'