Democrats may fall short of securing the minimum number of Republican senators needed to bring new witnesses into President Trump's impeachment trial, 10 senior staffers to key Senate Republicans tell Axios. The big picture: As of Thursday night, the prevailing view emerging among Republican Senate aides was that Democrats — who need four GOP senators' votes and not to lose any from their own party — will struggle to get more than three.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.