The number of Senate Republicans who seemed open to including witnesses in Donald Trump's impeachment trial were quite limited. With so much of the 53-member GOP conference allied with the White House, only a handful of senators were considered open-minded enough to be in the mix.

Among them was Sen. Lamar Alexander (R-Tenn.) -- a Republican whose lengthy career in public office is nearing its end, and a man with a reputation as an "institutionalist." If anyone in the chamber could appreciate the importance of a proper impeachment trial, and the damage it would do to the Senate if it were to cover up a president's misdeeds, it's the senior senator from Tennessee.

It's this background that made his decision that much more dejecting.

Sen. Lamar Alexander, R-Tenn., a key impeachment swing vote, announced Thursday that he will not join Democrats in voting to call witnesses in President Donald Trump's Senate impeachment trial, suggesting that there may not be enough GOP votes for the trial to advance to that next stage. "I worked with other senators to make sure that we have the right to ask for more documents and witnesses, but there is no need for more evidence to prove something that has already been proven and that does not meet the U.S. Constitution's high bar for an impeachable offense," he said on Twitter.

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As Alexander sees it, Trump did what he's accused of doing. Indeed, while the president continues to describe his antics as "perfect" and "beautiful," the Tennessee Republican conceded that Trump's actions were "inappropriate" and that the House impeachment managers successfully proved their case.

Alexander just doesn't much care. He read the call summary, heard the arguments, weighed the evidence, and concluded that Trump is guilty -- of an offensive that isn't especially important. It's why, in the senator's mind, there's no need for any kind of accountability.

It may be tempting to think that Alexander caved to partisan pressure, but that's probably not quite right. The Tennessean is nearly 80 years old; he'll never again seek elected office; he's independently wealthy; and he's in a position in which the White House and GOP leaders appear to have effectively no leverage over him. Alexander was free to vote his conscience.

And his conscience led him to disregard the misdeeds of a president whose guilt he considers obvious. Alexander not only won't vote to convict Trump and remove him from office, he believes the right course of action is to make this the first impeachment trial in American history in which the Senate doesn't even hear from witnesses.

Alexander has been a two-term governor, a cabinet secretary, a competitive presidential candidate, and a three-term senator, but it's likely that his decision on Donald Trump's fate will be the one that helps define his career. With his legacy on the line, the Tennessean made a choice that will likely haunt him indefinitely.

As for the state of play on Capitol Hill, in order for the impeachment trial to hear witness testimony, including John Bolton's perspective, proponents will need 51 votes in a chamber in which the Democratic conference has 47 members. Sen. Susan Collins (R-Maine) issued a statement last night in support of hearing from witnesses, and it's likely that Sen. Mitt Romney (R-Utah) will do the same.

Sen. Lisa Murkowski (R-Alaska) -- by some accounts, the only undecided Republican -- is expected to announce her decision this morning. If she votes with her party, there will be no witnesses and the entire trial may come to a rather abrupt end later today.

If the Alaskan votes to hear witness testimony, the Senate may end up in a 50-50 tie. At that point, things may get a little tricky, though as NBC News' report on this added, a tie vote will probably mean failure,

Watch this space.

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