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– A potentially pivotal disclosure from a book draft by John Bolton, President Donald Trump's former national security adviser, about the Ukraine affair has heightened the question of whether the Senate will call Bolton as a witness in the impeachment trial.

But the rules for how a witness might be subpoenaed are murky, with gaps in the written procedures and only a few precedents to look to.

What is the clearest way for the Senate to subpoena a witness?

With a 51-vote majority.

Under the special procedures for the Trump impeachment trial that the Senate approved this month, after Trump's defense team finishes its presentation and there is a period of questions, the Senate will decide whether it wants to hear from witnesses. A motion to hear witness testimony can pass with a simple-majority vote.

Assuming all 47 members of the Democratic caucus stick together, that would require at least four Republican senators to defy Trump and vote for a motion to subpoena Bolton. Political observers have focused on Sens. Lamar Alexander, Susan Collins, Lisa Murkowski and Mitt Romney as Republicans to watch.

What if the Senate splits, 50-50? In theory, Chief Justice John Roberts could decide.

When a vote on a motion ends in a tie, the motion fails. But during the first presidential impeachment trial — the unsuccessful effort to remove Andrew Johnson in 1868 — Chief Justice Salmon P. Chase twice cast tiebreaking votes on motions as the presiding officer. They both passed.

The Senate's general rules for impeachment trials are silent about whether the chief justice may cast tiebreaking votes on procedural motions, and the current version — last revised in 1986 — does not address that question. A resolution that the Senate approved this month laying out specific procedures for the Trump trial is also silent about ties.

To be sure, both of the procedural motions that Chase voted on were far less momentous than calling Bolton as a witness would be.

May Roberts decide on his own, without any vote?

It's unclear.

In an opinion article published Monday by the New York Times, three legal experts declared that he could. They argued that Roberts, as the presiding officer, has unilateral power to subpoena witnesses because a clause in the 1986-era rules for impeachment trials says presiding officers may issue orders on their own.

There are several reasons for caution about this idea. First, even if this interpretation of the rules were correct — and even if Roberts were willing to subpoena Bolton on his own — a subpoena, by itself, would merely compel Bolton to show up physically. The 1986 rules also empower a Senate majority to decide that information is irrelevant and should not be admitted as evidence. That said, a subpoena from Roberts could increase momentum in favor of testimony.

Second, Roberts might reject the idea that he has the power to subpoena Bolton unilaterally were House impeachment managers to propose it, said Michael Davidson, who served as the Senate legal counsel from 1979-95. Even if the parliamentarian or Roberts were to say otherwise, Davidson added, a Senate majority could always vote to override them.

Finally, the opinion article said that changing Senate rules requires a two-thirds supermajority, and the Trump trial resolution fell short. But a simple majority of senators can change its rules, Davidson said. A two-thirds vote would be required to break any filibuster blocking an up-or-down vote on a rules change, but Democrats did not filibuster the Trump trial resolution.