In the tweet, Trump said Gillibrand, D-N.Y., asked him for campaign donations before he was elected president. He sent the post after Gillibrand called for the president to resign due to allegations of sexual misconduct.
"Lightweight Senator Kirsten Gillibrand, a total flunky for Chuck Schumer and someone who would come to my office "begging" for campaign contributions not so long ago (and would do anything for them), is now in the ring fighting against Trump. Very disloyal to Bill & Crooked-USED!" Trump wrote.
Gillibrand responded to the tweet at a news conference on Tuesday, condemning Trump's remark that she "would do anything" for donations.
"It was a sexist smear attempting to silence my voice, and I will not be silenced on this issue. Neither will the women who stood up to the president yesterday," she said.
Three women who accused Trump of sexual misconduct held a news conference in New York on Monday, calling for Congress to "put aside party affiliations and investigate Trump's history of sexual misconduct."
"Their voices also will not be silenced and neither will the millions of women and men who have marched against the president and his policies," Gillibrand said of the three women.
The White House denied Gillibrand's assertion that the tweet was sexist, stating it could only be interpreted as innuendo "if your mind is in the gutter."
White House press secretary Sarah Sanders said Trump's comment that Gillibrand "would do anything" for campaign donations was intended as a critique of the current campaign finance system and special interests. She said it is not sexist because the president has used the language in reference to male and female lawmakers.
"He's talking about the way our system functions as it is," she said. "Politicians repeatedly beg for money ... there's no way it's sexist at all."
"Are you really trying to bully, intimidate and slut-shame [Gillibrand]? Do you know who you're picking a fight with? Good luck with that [Trump]," she wrote.
Sen. Ron Wyden, D-Ore., said the sexist behavior was in Trump's nature.
"He's a misogynist and admitted sexual predator and a liar. The only thing that will stop him from attacking us, because nobody is safe, is his resignation," Wyden said.