There seems to be some sentiment in the U.S. Senate, even among a few Republicans, for calling former national security adviser John Bolton as a witness in the impeachment trial of President Donald Trump.

Fine. But doing so ought to open a Pandora’s Box on which some Democrats would prefer to keep the lid closed.

Bolton is making headlines because of his new book, in which he writes that Trump told him he planned to withhold U.S. military aid from Ukraine unless that country’s leaders opened an investigation into Joe Biden — who, at least for now, appears to be the leading Democratic Party contender for president.

Clearly, Democrats in Congress who have been trying to claim Trump’s scalp since he won the 2016 election see Bolton as some sort of smoking gun. Just days ago, they felt the same about Lev Parnas, who also claims to have information damaging to the president.

But are either of them reliable witnesses? Perhaps not. Bolton may have an ax to grind because Trump fired him. Parnas faces federal campaign finance charges and may be angling for leniency.

Still, the very fact both have made their claims makes them appealing to Democrat lawmakers.

Fine. But if a witness list is to be prepared, it needs to include both former vice president Biden and his son, Hunter.

Hunter Biden’s lucrative connection with Burisma, the Ukrainian natural gas company, ought to be explored. So should his father’s action, while vice president, in telling Ukrainian leaders that unless they fired the country’s top prosecutor, they would not receive $1 billion in promised U.S. aid.

Trump’s defenders insist he was right to insist that Ukraine investigate the Bidens. If senators of both political parties want to get to the bottom of allegations against the president, that side of the story needs to be explored, too.