There are several reasons why Republicans are reluctant to allow Bolton to testify. Various process arguments have been made about the lack of witnesses at the Clinton impeachment trial and the fact that House Democrats could have called Bolton and other witnesses but chose to rush through impeachment instead. Allowing Bolton to testify would, under this view, be rewarding Democrats’ shenanigans and allowing them to have it both ways. I agree that Democrats should not have rushed impeachment and that they should have fought to compel witness testimony. But when the impeachment proceedings were underway in the House, Bolton’s position was that he would testify if a court broke the impasse between the legislative and executive branch and ruled that he could testify. Now, he’s arguing that he’s willing to testify before the Senate without waiting for the courts to weigh in. That’s an important distinction.
In reality, Republicans are likely worried about introducing a late wild card into the impeachment process. As things stand, impeachment has been largely a bust for Democrats, having not won over voters who did not already object to Trump. The president’s approval ratings have been essentially unchanged since the Ukraine story broke in September, despite months of wall-to-wall coverage. Barring anything unexpected, Republicans will be able to acquit Trump within a few weeks and move on with reason to believe the Ukraine story will be largely forgotten by November. It’s not at all clear that Bolton’s testimony would be harmful to Trump. He’s also spent decades in the conservative movement and isn’t likely to want to burn all of his bridges by offering bombshell testimony that would take down a sitting president who has near-universal approval within the Republican Party. However, Republicans are likely reluctant to introduce an unknown when it feels as if they’re otherwise home free.