Leahy shared his concerns over confirming a conservative judge who would not check the power of the executive branch.
"At a time when the president is seizing unprecedented power, the Supreme Court needs to act as a check and to provide balance," said Leahy in a speech at Georgetown University's law school on Thursday. "Based on the hearing and his record, I have no confidence that Judge Alito would provide that check and balance."
Edward Kennedy of Massachusetts, Barbara Mikulski of Maryland and Max Baucus of Montana have announced they will vote against Alito. Most of the other Democrats in the Senate appear certain to follow their lead. Only one of the Senate's 44 Democrats, Sen. Ben Nelson of Nebraska, has publicly announced he will vote in favor of the nominee.
Alito would replace retiring Justice Sandra Day O'Connor who has often been the swing vote on the nine-member court on controversial topics such as abortion, civil rights and other social issues.
Senate Minority Leader Harry Reid said in an interview on Wednesday with The NewsHour with Jim Lehrer that it would be a stretch for him to confirm Alito because of this decisive role O'Connor played in the court's decisions.
"We feel that there should have been somebody like Sandra Day O'Connor replaced," said Reid. "The Judicial Committee established one thing: Samuel Alito is no Sandra Day O'Connor."
To this date, no Democrat has threatened a filibuster and Sen. Reid said that while some members are pushing for it, it is still too early for Democrats to make that decision.
After last week's hearings, Senate Judiciary Committee Chairman Arlen Specter predicted a party-line vote in both the committee and the full Senate.
In their weekly Political Wrap, NewsHour analysts Mark Shields and David Brooks agreed with Specter's statements.
"I think given the lines of division and the firmness of positions in Washington right now," said Shields. "That which is unthinkable in the case of Sam Alito -- an honorable and exemplary man -- wouldn't change votes."
Brooks defined the split as a disagreement over a woman's right to have an abortion: "Executive power is still an important part of this issue, but it is about abortion. And as long as we don't have an abortion debate in this country, we are going to have these sorts of issues which are really all about abortion in the attempt to target and destroy people who happen to disagree with you on abortion."
The Judiciary Committee is expected to vote on Wednesday followed by a debate and vote in the full Senate later in the week.