President Trump is poised to win his long-expected acquittal in the Senate impeachment trial as soon as tonight, after Sen. Lamar Alexander's dramatic 11th-hour announcement that he'll vote against calling new witnesses.

The state of play: The big question is no longer whether the Senate will sink this afternoon's witness vote, but rather how long it will take to deliver a final verdict on Trump after the vote fails.

Without Alexander, it's nearly certain that there won't be enough votes for the motion to call witnesses — short of either a surprise Republican vote in favor of it or Chief Justice John Roberts breaking a 50-50 tie. Neither is likely.

Susan Collins announced she'll vote for witnesses, and Mitt Romney is considered a likely vote for witnesses as well.

Lisa Murkowski is expected to announce her decision today.

Driving the news: Alexander — a key Tennessee Republican — announced late last night that he'll vote against witnesses because "there is no need for more evidence to prove something that has already been proven and that does not meet the United States Constitution’s high bar for an impeachable offense."

He said it was "inappropriate for the president to ask a foreign leader to investigate his political opponent and to withhold United States aid to encourage that investigation" — but added that the election is the appropriate way for Americans to decide what to do about it.

Alexander met privately with Murkowski prior to making his decision. The two then asked a joint question in the trial, just an hour before the Q&A session ended, that seemed to capture their ultimate play.

Even if former national security adviser John Bolton testifies that Trump linked Ukraine aide to investigations of the Bidens, they asked, "isn't it true that the allegation still would not rise to the level of an impeachable offense?"

The question was directed to Trump's counsel, a telling move given they could spin it to their advantage.

Between the lines: The buzz on the Hill as senators shuffled out of the Capitol last night was that the two Republicans have agreed to tie their vote together to avoid a 50-50 stalemate on the witness vote, which would put Roberts in an incredibly tough position. Senators from both parties have said they hope it never reaches that point.

What's next: The Senate reconvenes at 1 p.m. today and will begin up to four hours of debate, evenly divided, over the witness vote.

What happens after that is a bit more complicated. If the vote fails, as it's expected to, Republicans are eager to move to a final verdict as quickly as possible.

But several senators will likely want to first huddle together to discuss next steps — either behind closed doors, or they could vote to do so in an open setting — and use the opportunity to share how they came to their decision.

After "jury deliberations," as several senators described it, McConnell will likely introduce a motion to move to closing arguments from House managers and Trump's defense team. It's unclear how long they will be given to do so.

But some Democrats are weighing the possibility of offering amendments to the motion, forcing senators to go on the record with their votes and further delaying the impeachment process.

Once the potential amendment process is over, they will vote on whether to acquit Trump or remove him from office.

What we're hearing: "Say they do closed-door deliberations for a couple hours. That would be short, because it's 100 senators. Even if only half of them want to speak, it's a long time," a Republican leadership aide told Axios.