Photos show the group of demonstrators arriving outside the Supreme Court on Capitol Hill on Sunday, dressed in red robes and wearing white bonnets.
Barrett, a conservative, would shift the balance on the Supreme Court significantly to the right, from five-four in favour of conservatives to six-three.
The hearings have drawn criticism as they are beginning just 22 days before the US general election.
In 2016, Senate Republicans refused to hold confirmation hearings for Merrick Garland, Barack Obama’s Supreme Court pick to replace Antonin Scalia, arguing that those should take place after the presidential election.