Roberts has the power under longstanding Senate rules to “rule on all questions of evidence.” That means he could, in theory, allow the House Democrats serving as prosecutors to call the witnesses they want. Some lawyers suggest this could allow Democrats to bypass White House impediments to getting testimony from high-level administration officials.

But a ruling favoring Democrats, who want to present witnesses they say would describe Trump blocking military aid to Ukraine to gain political help at home, could prompt a vote to override. And Roberts will “want to avoid circumstances where the Senate is being asked to overrule him,” said Michael Gerhardt, a University of North Carolina law professor and author of “Impeachment: What Everyone Needs to Know.”

The standing rules give Roberts an escape clause: He can “submit any such question to a vote of the Members of the Senate.”…

McConnell’s approach to the trial could rest in part on whether the Senate’s 53 Republicans are united. If they are, they can make their will felt in setting rules. If not, McConnell has a greater incentive to work something out with Senate Democratic leader Chuck Schumer of New York.