"A picture of Sam Alito is emerging that may explain why the extreme right-wing is popping champagne corks," Reid, D-Nev., said in a Senate speech, referring to a 20-year-old document in which Alito asserted "the Constitution does not protect a right to an abortion."
Abortion rights activists are concerned that Alito and recently confirmed Chief Justice John Roberts would move the Supreme Court to the right and perhaps overturn the landmark 1973 Roe v. Wade decision that established abortion rights.
"I will work to ensure that Judge Alito is treated with civility and respect," Reid said. "But there is nothing disrespectful about an open and fair-minded review of a nominee's approach to the Constitution and his commitment to core American values like equality, privacy and fairness."
Bush nominated Alito last month as the replacement for retiring Justice Sandra Day O'Connor, who has been a crucial swing vote on contentious issues, including abortion, during her 24-year high court career.
Alito was Bush's second choice after White House counsel Harriet Miers withdrew under withering criticism from conservatives.
"Harriet Miers was forced to withdraw by conservative activists who want to change the legal landscape of America," Reid said. "They decided she was inadequately radical or insufficiently aggressive for their purposes, so they gave her the boot."