Ms Rice said the senator had “put party and politics over her own stated principles” of supporting equal rights and legal abortions.
“I think in a way that I really regret saying, she has betrayed women across this country,” she said at the The New Yorker Festival in New York.
Ms Rice, who also served as US ambassador to the United Nations (UN) under the Obama administration, said she would decide whether to run for the Senate from Maine in 2020 after next month’s midterm elections.
It comes after Dr Christine Blasey Ford testified to the Senate Judiciary Committee that Mr Kavanaugh sexually assaulted her at a house party when the two were teenagers.
Shortly before the Supreme Court vote, Ms Collins said: ”I do not believe that these charges can fairly prevent Judge Kavanaugh from serving on the Court.”
However, she described Dr Ford’s testimony as “sincere, painful and compelling.”