Kagan accused her conservative colleagues of doing Congress' work for them, saying "this Court has no right to remake" a key section of the Voting Rights Act.
The case comes as Republicans in some states are pursuing new restrictions after Trump made false claims of widespread voter fraud.
The liberal justices dissented as the Supreme Court ruled against extending Wisconsin’s deadline to count mailed-in ballots received after Election Day.
Judge Amy Coney Barrett would replace Justice Ruth Bader Ginsburg, who is best-known for fighting for women’s rights and equality.
"Ask yourself one simple question: Is this how American democracy is supposed to work?" the Supreme Court justice said in June.
Attendees at Monday's casket-viewing praised the late Supreme Court justice as a "brilliant man" who was devoted to the rule of law.
Kagan dissented strongly in the June 27 case and said Thursday she was writing for the future.
In a blistering dissent read from the bench, the Supreme Court justice said extreme gerrymandering could render elections "meaningless."
Chief Justice John Roberts joined the court’s four liberals in siding with inmate Vernon Madison.
The congressman has a long history of incendiary and offensive remarks.