Democrats would need to peel off four Republican senators to successfully call a witness or compel the administration to hand over Ukraine-related documents.

McConnell told Fox News Radio that opening the door to witnesses would pave the way for a "kind of mutual assured destruction," adding that Republicans would "want the whistleblower. We're going to want Hunter Biden."

"You can see here that this would be a kind of mutual assured destruction episode that would go on for a long time," McConnell said.

Alexander Bolton contributed.