On the most important question of the trial — whether to subpoena witnesses — the 79-year-old Tennessee Republican senator is a wild card. Privately, senior Senate Republicans expect the vote to seek witness testimony to fail, but they are watching Alexander and several other Republicans closely. And wherever Alexander comes down is almost sure to be the majority position in the Senate.

Three GOP senators have expressed some level of support for calling witnesses, and if they joined all Democrats, it would result in a 50-50 tie and likely be defeated. Unless Chief Justice John Roberts shocked Washington by wading in with a tie-break, Democrats need one more Republican to break ranks and upend GOP plans for a swift Trump acquittal.

That’s got both parties eagerly eyeing Alexander. He’s a retiring defender of the Senate as an institution who’s occasionally bucked his party, but he also counts Majority Leader Mitch McConnell as a longtime ally. He’s more hesitant to criticize Trump than are some other Republicans, but he also has said it was “inappropriate” for Trump to ask foreign governments to investigate his political opponents.