Trump called Gillibrand a "lightweight" and a "total flunky" for Senate Minority Leader Chuck Schumer, New York's other senator. He also claimed she used to come to his office "begging" for campaign contributions, cryptically adding the senator "would do anything for them." It is unclear what he meant by that statement.
Trump also called her "disloyal" to the Clinton family but did not elaborate on why.
Gillibrand shot back on Twitter, saying, "You cannot silence me or the millions of women who have gotten off the sidelines to speak out about the unfitness and shame you have brought to the Oval Office."
Later Tuesday, she called Trump's tweet a "sexist smear" and said it would not silence her or the women who "stood up" to Trump on Monday by calling on Congress to investigate misconduct claims against him.
Speaking to reporters later Tuesday, Schumer called Trump's tweet "nasty" and "unbecoming of a president."
Trump made donations to Gillibrand's campaign committee in 2007 and 2010 totaling $5,850. His daughter and advisor, Ivanka, contributed $2,000 to the campaign committee in 2014.
The public confrontation started Monday, when Gillibrand told CNN that Trump "should resign" due to the sexual misconduct allegations against him. If Trump does not resign, Congress should investigate the "credible" and "numerous" accusations against the president, she said.
More than 50 Democratic women in Congress on Monday asked the House Oversight Committee to investigate Trump, according to NBC News. Three men in the Senate Democratic caucus —Bernie Sanders, Cory Booker and Jeff Merkley — have also said Trump should think about resigning or have outright urged him to step down.
At least a dozen women have accused the president of sexual misconduct. On Monday, three of those women urged Congress to investigate the president.
In a separate tweet Tuesday morning, Trump said he is the target of "false accusations and fabricated stories of women who I don't know and/or have never met."
White House press secretary Sarah Sanders said Monday that Americans knew about the allegations last year and had the chance to judge Trump when they elected him president.
Other Democratic women in the Senate — Elizabeth Warren of Massachusetts and Maggie Hassan of New Hampshire — tweeted support for Gillibrand following Trump's jab.
Asked later Tuesday whether Trump's tweet was sexist, White House press secretary Sarah Sanders said the president has used the "same terminology" to describe both men and women. She added that Trump was attempting to express that he "can't be bought."