Harris took to Twitter late on Monday, after Barrett was sworn in at a ceremony at the White House late on Monday, taking the first of the two oaths required for her to officially become a Supreme Court Justice.
Earlier on Monday, the US Senate confirmed Judge Amy Coney Barrett's nomination to be a Supreme Court justice by a 52-48 vote, strictly along partisan lines, filling the vacancy left by the late Ruth Bader Ginsburg.
House Speaker Nancy Pelosi said in a statement commenting on Barrett's confirmation that it was a "manipulation" that threatens the rights of the American people and that the United States now has "a radical Republican court".
She added that Congress "will have to reverse the damage" and "defend pre-existing condition protections together with every other benefit and protection of the Affordable Care Act".
"We must have a Congress that is prepared to remedy what the Supreme Court will do to undermine the health, financial security and well-being of American families", Pelosi said.
The US Supreme Court had a vacant spot after Ginsburg, 87, died in September of complications from metastatic pancreatic cancer at her home in Washington.
After the Monday Senate vote, Barrett took the constitutional oath at the White House swearing-in ceremony. On Tuesday, Chief Justice Roberts will administer Barrett's judicial oath and she will officially be a Supreme Court Justice and on the job.
US President Donald Trump had wanted to fast-track Barrett's confirmation because he feels the looming presidential election could end up being decided by the Supreme Court. The Democrats had been pushing back, arguing that the winner of the election should be the one to fill the vacancy.
The Supreme Court is expected to take up the latest lawsuit challenging the constitutionality of the Affordable Care Act. The court has previously rejected other constitutional challenges to the law, widely known as Obamacare.