Sen. Rand Paul, R-Ky., on Monday said if his fellow Republicans allow Democrats to have the witnesses they demand in the impeachment trial of President Donald Trump, he will demand Hunter Biden testify.

Paul tweeted:

"My colleagues can't have it both ways. Calling for some, while blocking others. If we are going to give a platform to witnesses the Dems demand, I look forward to forcing votes to call Hunter Biden and many more!"

Senate Majority Leader Mitch McConnell, R-Ky., who also opposes witnesses in the trial, has warned against them for just such a tit-for-tat reasoning.

McConnell told Fox News Radio in December that a calling of witness by Democrats coud lead Republicans to want Hunter Biden and even the whistleblower who's identity has never been officially revealed.

Such a move would be a "kind of mutual assured distraction" that might be unending, McConnell warned.

Other Republicans, such as Maine's Susan Collins and Utah's Mitt Romney, have been more open to possibly allowing witnesses. Democrats needs only four Republicans to cross over and vote with them to get their wish. CBS News reported Monday they likely have found four Republicans to join them.

Romney has said he would like to hear former National Security Adviser John Bolton testify. Bolton has said he is willing to testify if subpoenaed.