Plenty of elected Republican leaders have criticized their party's presumptive nominee Donald Trump, especially over the past few days as Trump has launched a barrage of racist attacks on federal judge Gonzalo Curiel. But since most Republicans continue to endorse Trump, their criticism fell a little flat: It left the impression that Republicans could forgive Trump's racism because at least he wasn't Hillary Clinton.

But some Republicans are beginning to draw a line in the sand. This week, Sen. Lindsey Graham (R-SC), who hasn't endorsed, called on his colleagues to withdraw their endorsements. And Sen. Mark Kirk (R-IL) proceeded to do just that.

"I cannot and will not support my party's nominee for president," Kirk told reporters. A few minutes later, he tweeted this:

Given my military experience, Donald Trump does not have the temperament to command our military or our nuclear arsenal. — Mark Kirk (@MarkKirk) June 7, 2016

Kirk is a Republican senator in a blue state (Illinois) who has trailed Democratic challenger Tammy Duckworth in polls. So he has more incentive than the average Republican to disavow Trump. You see some version of this every cycle, where candidates in tough races distance themselves from their party's presidential nominee. (Or even sitting president — when Alison Lundergan Grimes ran for Kentucky's Senate seat on the Democratic ticket in 2014, she famously refused to say whether she'd voted for Obama.)

But Kirk's un-endorsement is different for two important reasons. For one thing, there's a difference between simply not endorsing a nominee (which is what people in Kirk's position typically do) and making an endorsement, then calling backsies. But since March, Kirk has said that if Trump is the nominee, he'll support him for president. Now he's withdrawing that commitment — which might break the seal for other Republicans considering abandoning Trump.

The other difference is why Kirk is un-endorsing his party's candidate. He's not just chalking it up to differences of opinion — he's saying that Trump is temperamentally unfit for the presidency. He's even raising the specter of Trump casually getting involved in nuclear war, which is something even Trump's most gimlet-eyed critics consider an unlikely possibility.

That's a very serious accusation to make about the man your party wants to send to the White House! But what's more alarming is the near-certain possibility that there are other Republicans who agree with Mark Kirk that Trump is unfit to be president — but who don't have Kirk's blue-state incentives to say so out loud.

Donald Trump's rise is a scary moment in America