The battle Trump launched with Gillibrand is not politically advisable, as The Fix's Aaron Blake points out. But based on his past behavior, it was entirely predictable.
Here are a couple likely reasons Trump is attacking Gillibrand and leaving the other senators alone.
She's a woman criticizing him
Four Democratic senators had already called on Trump to resign by the time Gillibrand went on CNN International Monday and said he should step down.
Those four senators are all men, though.
Trump picks fights with men and women. But he has a history of lashing out at his female opponents in particularly gendered ways. See Hillary Clinton. Ted Cruz's wife. MSNBC host Mika Brzezinski. And now, Gillibrand.
The president has a tendency to reduce women to centuries-old stereotypes that squarely fit the definition of sexism, Kelly Dittmar, a Rutgers University professor and women in politics expert, told me after Trump attacked Brzezinski's appearance when the host criticized his policies.
“Trump has played the gender card all along and played into very stereotypical tropes,” Dittmar said, adding: “like characterizing women's values and intelligence by their appearance and also calling women things like 'crazy,' which has been done to women for centuries when they've spoken truth to power.”
She might try to run against him
Senate Democrats criticizing the president is nothing new. But Trump may be particularly sensitive to Gillibrand, because she is thought to be considering a challenge to Trump in 2020.
But so is every other Democrat and liberal celebrity with a Wikipedia page, basically. (Seriously. Blake had to cut off The Fix's list of Democrat 2020 contenders at 15.)
To my earlier point about Gillibrand's gender: Sen. Cory Booker (D-N.J.) is also a potential challenger to Trump, who recently said the president should resign. Booker didn't just say Trump should resign — he said it while campaigning for the Democrat in the Alabama Senate race over the weekend. That didn't get a peep out of Trump.
The other possible conclusion here is that Trump considers Gillibrand more of a threat than some of the other contenders. Specifically when it comes to sexual harassment. She has made a name for herself trying to prevent sexual assault in the military, and she was the first Senate Democrat to ask Sen. Al Franken (D-Minn.) to resign over groping allegations.
Gillibrand and Trump have a history
Strip out the sexually suggestive innuendos in Trump's tweets, and you're left with this: Gillibrand asked me for money when she was running for the Senate, so how dare she criticize me now.
Gillibrand said that Trump did support her first campaign for Senate, in 2008. She's from New York, and Trump was a New York donor.
But Trump never seemed to be a major donor to Gillibrand. As The Washington Post's Ashley Parker and John Wagner pointed out, Open Secrets, a nonprofit organization that tracks campaign contributions, calculates that Trump has donated $5,850 to Gillibrand's campaign since 1996. That's less than he has given to other New York politicians, including Democrats.
She was on his TV this morning
Gillibrand was scheduled to hold a news conference Tuesday morning at the Capitol to talk about legislation aimed at making tractor-trailer truck crashes less deadly.
Yes, that legislation had nothing to do with Trump and sexual harassment. And no, it wasn't likely to be a major media event. But it did mean that Gillibrand would be in front of the cameras again, a day after she said Trump should resign.
It's probably not a coincidence, then, that Trump's tweet came a couple hours before Gillibrand was scheduled to talk.
But here's where we get back to the part about Trump's attack on Gillibrand being politically inadvisable.
He had lit the fuse, and Gillibrand responded in a headline-grabbing way. At that news conference for tractor-trailer truck safety, she flat-out called the president's remarks sexist: “It was a sexist smear attempting to silence my voice. I will not be silent on this issue, neither will women who stood up to the president yesterday and neither will the millions of women who have been marching since the Women’s March to stand up against policies they do not agree with.”