When it comes to impeachment, the Republican Senate, resting on the bedrock of Donald Trump’s base, has been viewed as an unbreachable wall. But what would it look like if it started to crack? The first sign might be an ominous quiet. The emergence of a second intelligence-agency whistle-blower, this one reportedly with a firsthand account that Trump withheld military aid from Ukraine to pressure its government to investigate Joe and Hunter Biden, complicates the president’s deep state conspiracy defense—and the senators on whom Trump’s political survival rests are mostly lying low. “The messaging is missing this time,” a former West Wing official told me. Even Senate Majority Leader Mitch McConnell said last week that he would be forced to hold a Senate trial if the House impeached Trump. “There’s been a real increase in nervousness over the past three or four weeks,” a prominent GOP member told me. “Everybody sees what Trump did as such a clear abuse of power,” said another prominent Republican. “Whether it’s criminal or not is another issue. But it’s so blatantly over the line.”

There’s no flood of GOP refugees yet—but there’s unmistakable drift. Trump’s approval rating among Republicans is 87%, a drop of four points since mid September, according to the latest Gallup tracking poll. Meanwhile, Trump’s firewall at Fox News, a vital bulwark that protects his GOP base, has also shown signs of cracking. Tucker Carlson conceded in an op-ed that Trump’s actions were indefensible, though not impeachable. Fox News legal analyst Judge Andrew Napolitano called Trump’s Ukraine actions “criminal and impeachable behavior” last week. Behind the scenes, Fox board member Paul Ryan has privately told Fox Corp CEO Lachlan Murdoch to cut ties with Trump.

In the Senate, Ben Sasse and Susan Collins have made their usual equivocal noises—but not surprisingly, its Mitt Romney, longtime Trump antagonist and sometime suck-up, who’s become the standard-bearer, leading to questions as to what his game is. According to sources, donors have in recent days called the Utah senator and encouraged him to run against Trump in the primary. “There is a half-billion dollars on the sidelines from guys who are fed up with Trump,” a GOP donor told me. Their hopes were raised when Romney attacked Trump on Friday, tweeting: “By all appearances, the President’s brazen and unprecedented appeal to China and to Ukraine to investigate Joe Biden is wrong and appalling.” Former White House communications director turned Trump antagonist Anthony Scaramucci tweeted a poll showing Romney beating Trump 55% to 37% in a hypothetical primary. Trump fired back on Saturday. “Mitt Romney never knew how to win,” Trump tweeted. “He is a pompous ‘ass’ who has been fighting me from the beginning, except when he begged me for my endorsement for his Senate run (I gave it to him), and when he begged me to be Secretary of State (I didn’t give it to him). He is so bad for R’s!”

According to people close to Romney, he’s firmly decided against primarying Trump, an enterprise he believes to be a sure loser given Trump’s enduring GOP support. Romney has also told people that, as an unsuccessful two-time presidential candidate, he’s the wrong person to take on Trump. Instead, a Romney adviser told me, Romney believes he has more potential power as a senator who will decide Trump’s fate in an impeachment trial. “He could have tremendous influence in the impeachment process as the lone voice of conscience in the Republican caucus,” the adviser said. In recent days, Romney has been reaching out privately to key players in the Republican resistance, according to a person briefed on the conversations. “Romney is the one guy who could bring along Susan Collins, Cory Gardner, Ben Sasse. Romney is the pressure point in the impeachment process. That’s why the things he’s saying are freaking Republicans out.” (Romney, through a spokesperson, declined to comment.)

-snip-