Senators who want Roberts to intervene have pointed to the precedent set by then-Chief Justice Salmon P. Chase in the 1868 impeachment trial of President Andrew Johnson.

Chase broke two ties on procedural deadlocks, including a vote on whether he could break ties.

Senate Democratic Leader Charles Schumer (D-N.Y.) has focused his demands on four witnesses: Bolton, Mulvaney, senior White House adviser Robert Blair, and senior Office of Management and Budget official Michael Duffey.

Republicans are pushing back, arguing Trump is expected to assert executive privilege to block the four witnesses Democrats have requested.