Senator Kirsten Gillibrand is firing back at President Trump and vowing to keep fighting him ... on the heels of him saying she'd "do anything" for campaign cash.

We got the U.S. Senator from New York Tuesday morning on Capitol Hill, just a few hours after the President tweeted and called Gillibrand a "total flunky" who had begged him for donations. The shot many call "slut-shaming" came when Trump claimed the Senator "would do anything" for his money.

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!

— Donald J. Trump (@realDonaldTrump) December 12, 2017

Gillibrand doesn't seem fazed, and dismissed Trump's tweet as a "sexist smear" -- and suggested he better not hold his breath waiting for her to back down.

Gillibrand called for 45's resignation Monday after Trump's accusers called for Congress to investigate him for sexual misconduct.