Why this matters: The calling of additional witnesses — in particular Trump's former national security adviser John Bolton — appears to be the Democrats' last hope of inserting even a slight detour into what currently seems like a straight road to Trump's acquittal.

• Without extra witnesses, Trump's impeachment could be over by the end of next week.

Details: Dems' initial targets included Susan Collins of Maine, Lisa Murkowski of Alaska, Mitt Romney of Utah, Lamar Alexander of Tennessee, Cory Gardner of Colorado, Thom Tillis of North Carolina and Joni Ernst of Iowa.

• These are senators who, for different reasons, had been seen as potentially persuadable to hearing more evidence beyond what the House Democrats gathered in their impeachment proceedings.
• But Tillis is now a no, and colleagues and aides also believe Ernst and Gardner will be.
• Among the remaining core of four, Alexander may be tougher to persuade than the other three, said current and former aides who have known him for years.
• Collins, Murkowski and Romney all have been clear they're open to witnesses, while, as Politico notes, Alexander has not.

What we're hearing: Alexander is extremely close to Senate Majority Leader Mitch McConnell, a key reason these associates are skeptical he'd ultimately cause problems for him.

• One GOP aide said their relationship is one of the closest colleague relationships McConnell has on the Hill.
• But Alexander's openness to bipartisan compromise and willingness to challenge Trump has made him distinctive in a deeply partisan era. He's retiring from his Tennessee seat in the fall and may have an eye toward his legacy.
• "Lamar is truly undecided on witnesses. Remember he is a lawyer himself and was a damn good one in private practice," a source familiar with his thinking told Axios. "You hear the House’s case. You hear the president’s response. You ask questions. Then you decide if witnesses are needed."

Between the lines: Just because some senators pushed to guarantee there would be a vote to witnesses if a majority supported that, it doesn't mean they'll vote to hear from witnesses when the time comes, a Senate GOP aide said.

• Murkowski indicated to reporters Thursday a frustration that House Democrats didn't want to take the time in court to try to force witnesses, but that if the Senate votes for witnesses that's likely where it would end up anyway.

Behind the scenes: At a private lunch on Thursday, chiefs of staff to Republican senators agreed that one moment in particular had backfired on Democrats: when House Judiciary chair Jerry Nadler declared on the Senate floor that any senator who refused to vote for more witnesses would "be complicit in the president's cover up."

• Republican senators reacted furiously to that suggestion — and it wasn't just the reliable Trump allies who expressed outrage: Collins told reporters she was "stunned" by Nadler's rhetoric, prompting her to send a note to Chief Justice John Roberts who later admonished both sides over their behavior.
• “I took it as offensive,” Murkowski told reporters Wednesday. “As one who is listening attentively and working hard to get to a fair process, I was offended.”
• Another moment that has been whipping up outrage inside the Republican conference was Schiff's suggestion that the 2020 election result cannot be trusted.
• Schiff said Wednesday that Trump's "attempt to use the powers of the presidency to cheat in an election" shows that his "misconduct cannot be decided at the ballot box—for we cannot be assured that the vote will be fairly won."

Worth noting: It's impossible to predict the thinking of senators who have remained largely inscrutable during impeachment.

• Informed predictions are one thing, but it's simply too soon to declare the witness vote a foregone conclusion, especially after the Collins curve ball earlier this week that forced McConnell to change the ground rules for the trial at the last minute.

• But Democrats plan to highlight how the report from the nonpartisan government agency frequently referred to as the "congressional watchdog“ clearly finds that the Trump administration violated the law.
• “This is a big part of our case,” a Democratic leadership aide told Axios. “It shows the extent to the president went to advance his scheme; he went so far to break the law. It's an important piece of evidence and only adds to the mountain and body of evidence that we already have."

What to watch: Whether new evidence uncovered by the House will be blocked from the Senate trial. The organizing resolution laying out the terms for the impeachment trial, released by Senate Majority Leader Mitch McConnell last night, left open the chance that the Senate could decline to review such evidence.

What we're hearing: Sources working with the president’s legal team told reporters Monday that the impeachment of Trump is illegitimate and historically aberrant because it doesn’t accuse Trump of committing a crime.

• One said: "We’re going to trial on a specific set of charges, spelled out in the charging document. The charging document doesn’t include anything about that GAO report. So in our view that’s not properly part of the accusation that‘s been brought to the Senate.”

Meanwhile, a Democratic aide working on impeachment said: "The president was so eager and so determined to turn the screws and ratchet up the pressure on Ukraine to do his political dirty work, that he was willing to break the law by withholding much needed security assistance from Ukraine.

• “That’s the reason why they don't want that document” to be considered, the aide said.

The backdrop: The GAO found that the White House Office of Management and Budget violated the law when it withheld military aid to Ukraine.

• The agency's report, which dropped just hours the Senate impeachment trial began last Thursday, determined that the OMB violated the 1974 Impoundment Control Act, which mandates that the White House and its agencies release funds appropriated by Congress.

Yes, but: The GAO's only tool to hold violators of the ICA accountable is to sue the administration to distribute the funds. OMB already released the aid to Ukraine in September.

Why it matters: They're banking on it to speed up the trial, according to people close to the president.

What we're hearing: White House counsel Pat Cipollone, who considers himself a civil libertarian, is expected to argue that the obstruction of Congress article is dangerous and could forever undermine the power of the executive office to protect privileged information.

• Cipollone will likely frame the Senate trial as a defining moment to set the precedent for executive privilege, especially on national security matters, per a source familiar with his thinking.
• This approach is something Cipollone is particularly proud of, and one that he is happy to test in court, the source said.
• The argument: Presidential claims of executive privilege are especially strong when they involve conversations about national security.
• Weakening that privilege would make presidents less candid when they seek counsel from their advisers on national security (think John Bolton).

The bottom line: Sources close to Trump's legal team have privately expressed confidence that former national security adviser Bolton will ultimately honor Trump's assertion of executive privilege.

Go deeper: Trump's concede-nothing defense